RIDING LIFT TABLE FOR DETECTION OF MINERAL SAMPLE BOX ON VERTICALLY EXTENDING RACK CROSS-REFERENCE TO RELATED PATENT APPLICATION This application claims the benefit of Korean Patent Application No. 10-2009-0097290, filed on October 13, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. FIELD OF THE INVENTION The present invention relates to a riding lift table for detection of a mineral sample box held on a vertically extending multilevel rack having a predetermined height, thereby enabling detection, management, recordation and so on of a mineral sample at the position of the rack, without directly lowering the mineral sample box on the ground. BACKGROUND Generally, when a borehole is drilled in the ground, a ground sampling process of soil and rock, among others, is necessarily and in advance performed to determine the parameters required for a construction design. This drilling investigation must be performed to get various geological stratum data, such as physical and chemical properties and stratified earth structures, among others, of the ground to be based for a construction structure. The ground samples of soil, rock and the like are collected and held in sample boxes. For management, the sample boxes are held in a plurality of racks or the 1 like which include multilevel selves and are structured to have a predetermined height. As the rack is installed at a predetermined height or higher, it is advantageous to hold many mineral samples or the like in the space with a predetermined height. However, much trouble is required to retrieve the mineral samples or the like from the rack installed at a predetermined height or higher. Moreover, there is a risk of an accident when retrieving the mineral samples or the like from the high rack. In a conventional manner, to retrieve/return the mineral samples from/on the rack installed at a predetermined height, a worker stands against the rack and has to repeatedly climb up/down the ladder. It is very difficult to raise and lower the sample boxes, mineral samples or the like in certain dimensions, using a ladder. When a worker retrieves/returns the sample box or the like positioned on the high rack without a ladder, the risk of an accident increases because the worker cannot directly check the situation on the rack or the condition in the sample box with the naked eye. When the worker picks up the sample box or samples from the rack, (s)he climbs down the ladder and puts down the sample box or samples on the floor to perform required work. After finishing the work, the worker again climbs up the ladder and returns the sample box or samples to the original position. In this conventional manner, it takes too much time for the work, thereby lowering the working efficiency, increasing the physical fatigue of the worker or manager and therefore decreasing the productivity. It is an aim of the invention to provide a riding lift table for detection of a mineral sample box which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides the consumer with a useful or commercial choice. 2 It will be clearly understood that any reference herein to background material or information, or to a prior publication, does not constitute an admission that any material, information or publication forms part of the common general knowledge in the art, or is otherwise admissible prior art, whether in Australia or in any other country. SUMMARY OF THE INVENTION To solve the above problems of the conventional art, it is therefore an object of the present invention to provide a riding lift table for detection of a mineral sample box held on a vertically extending rack, whereby a worker rides on the lift table, gets close to the rack where the mineral sample boxes are positioned and straightly performs various work on a workbench installed on the lift table, so that unlike the conventional art, the worker does not need to use any additional device (such as a ladder or the like) to retrieve the various mineral sample boxes positioned and held at different heights and put down the mineral sample boxes on the ground to perform required work; and whereby a height is adjustable so that the worker gets close to the mineral sample boxes, irrespective of the position of each mineral sample box. The above and other objects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof. Further, the objects and advantages of the present invention may be realized by the elements set forth in the claims and combinations thereof. In accordance with an exemplary embodiment of the present invention, there is provided a riding lift table for enabling detection, management and recordation of a 3 mineral sample box held on a vertically extending rack with multilevel shelves, comprising: an underframe with its bottom or sides at which a plurality of movable members are positioned, and its top on which a foldable chair and desk for work are installed; a workbench positioned horizontally on the top of the underframe; a link member installed between the underframe and the workbench, for moving the underframe vertically; a cylinder extended and contracted by a working oil supplied from an oil hydraulic pump, for moving the link member vertically; and a workbench control lever for operating the oil hydraulic pump, to supply the working oil to the cylinder. In accordance with the other exemplary embodiment of the present invention, there is provided a riding lift table for enabling detection, management and recordation of a mineral sample box held on a vertically extending rack with multilevel shelves, comprising: an underframe with its bottom or sides at which a plurality of movable members are positioned, and its top on which a foldable chair and desk for work are installed; an upperframe positioned horizontally on the top of the underframe; a link member installed between the underframe and the upperframe, for moving the upperframe vertically; a cylinder extended and contracted by a working oil supplied from an oil hydraulic pump, for moving the link member vertically; 4 a workbench control lever for operating the oil hydraulic pump, to supply the working oil to the cylinder; a height-control member installed on a top of the upperframe, for controlling a length in a vertical direction; and a workbench installed on the height-control member and moved vertically by the height-control member. Further, the chair for work comprises: a seat and a foldable backrest; a support operatively supporting the seat and including a bent part formed at its one end and fitted into a groove formed to be depressed from the top of the underframe; and a fixing member connected to the other end of the support by a hinge and covering and holding a side of the underframe, and the chair for work is attachable/detachable to/from the underframe. Further, the workbench comprises: a plurality of ball rollers installed so as to be rotatably and partially exposed on the top of the workbench where a workpiece is placed, for smoothly moving the workpiece; and a plurality of fence members installed so as to vertically stand at an edge of the workbench, thereby preventing the workpiece from falling off the workbench, and so as to move from a vertical position to a horizontal position to be level with the workbench on the same plane, thereby easily placing the workpiece. Further, each of the movable member comprises: a plurality of mobile units installed on the bottom of the underframe; and a plurality of stoppers each installed proximately to each of the mobile units, a length of the stopper controlled in a vertical direction by a rotation operation so as to be in contact with or spaced apart from the ground. 5 Further, each of the movable member comprises: a base with its one end fixed to the side of the underframe by a hinge; a first mobile unit formed at a bottom of one end of the base and always contacting with the ground; a second mobile unit formed at the other end of the base; and an additional cylinder with its one end fixed to the underframe and extended and contracted by the working oil supplied from the oil hydraulic pump, for pivoting the other end of the base so that the second mobile unit is in contact with or spaced apart from the ground. BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: FIG. 1 is a front view illustrating a lift table according to a first embodiment of the present invention; FIG. 2 is a front view illustrating a lift table according to a second embodiment of the present invention; FIG. 3 is a front view illustrating the operation of a height-control member shown in FIG. 2; FIGS. 4 and 5 are perspective views respectively illustrating the lift table of FIG. 1 before and after the operation; FIG. 6 is a perspective view illustrating a link member used in the lift table; FIG. 7 is a perspective view illustrating the height-control member of FIG. 3; FIG. 8 is a front view illustrating the operation of a fixing member used in the lift table; 6 FIG. 9 is a view illustrating that a chair for work is attached/detached to/from an underframe; FIG. 10 is a view illustrating the operation of a fence member in the lift table; FIG. 11 is a front view illustrating the operation of the fence member in the lift table; and FIG. 12 is a view illustrating the operation of a movable member in the lift table. [Brief description of reference numbers of major elements] 10: under frame 11: movable member 12: mobile unit 13: stopper 14: rotation knob 20: workbench 21: ball roller 22: fence member 23: supporting shaft 24: second combination member 25: extension tongue 26: fixing shaft 27: first combination member 28: tongue receiving opening 30: link member 40: upper frame 50: height-control member 60: cylinder 70: workbench control lever 71: oil hydraulic pump 80: chair 81: seat 82: backrest 83: support 84: bent part 85: groove 86: receiving part 90: desk DETAILED DESCRIPTION OF THE INVENTION It will be understood that, detailed constitution and arrangements of elements 7 described in the detailed description or illustrated in the drawings should not be construed as limiting the application of the invention. The invention may be embodied in many alternate forms and performed in various methods. The terms or words to describe the direction of an apparatus or element (for example, "front", "back", "up", "down", "top", "bottom", "left", "right" and "lateral", among others) are used to simplify the description of the invention. It will be, therefore, understood that these terms do not mean that the relevant apparatus or element shall be only in the specific direction. It will be further understood that, although the terms, "first", "second", etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another, not to indicate relative importance or purposes. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. To achieve the aforementioned objects of the present invention, the present invention has the characteristics as follows: The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. It will be understood that words or terms used in the specification and claims shall not be interpreted as the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention. 8 Accordingly, while example embodiments of the present invention are capable of various modifications and alternative forms, embodiments of the present invention are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the invention to the particular forms disclosed, but on the contrary, example embodiments of the invention are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. A riding lift table for detection of a mineral sample box held on a vertically extending rack according to the present invention will be described with reference to FIGS. 1 through 12. As illustrated, the riding lift table for detection of a mineral sample box held on a vertically extending rack comprises: an underframe 10, a workbench 20, a link member 30, a cylinder 60 and a workbench control lever 70. As illustrated in FIGS. 1 through 7, the underframe 10 has a flat shape with a certain thickness. A plurality of the constituent elements to be described later will be installed at the underframe 10. A plurality of movable members 11 are installed on the bottom or sides of the underframe 10. In a first embodiment, as illustrated in FIGS. 1 through 7, rotatable mobile units 12, such as driving wheels (which are applied to a first mobile unit 15 and a second mobile unit 16 to be described later), are respectively installed at the corner of the bottom of the underframe 10, so that the underframe 10 is movable. A stopper 13 is positioned proximately to each movable member 11. Like the movable members 11, the stoppers 13 are installed under the bottom of the underframe 10. The movable members 11 are structured to be movable vertically and to be in contact with the floor or to be spaced apart from the floor, depending on 9 the rotation operation of a rotation knob 14 to the right or left. That is, the underframe 10 is moved to a user's desired position by the movable members 11. Not to move the underframe 10 at the desired position, the user rotates the rotation knobs 14 of the stoppers 13 so that the stoppers 13 move down so as to be in contact with the floor (the ground). Each stopper 13 and each movable member 11 may be formed in a single body, at the user's choice. In a second embodiment, as illustrated in FIG. 12, the movable members 11 are installed on the sides of the underframe 10. The movable member 11 comprises: a base 14, a first mobile unit 15, a second mobile unit 16 and an additional cylinder 17. One end of the base 14 is fixed at the side of the underframe 10. The first mobile unit 15 is positioned under the bottom of one end of the base 14. The first mobile unit 15 rotates while supporting the underframe 10. The second mobile unit 16 is positioned at the end of the other end of the base 14. One end of the additional cylinder 17 is connected to an upper part of the other end of the base 14, and the other end of the additional cylinder 17 is fixed to the underframe 10. As the one end of the additional cylinder 17 connected to the upper part of the other end of the base 14 is extended/contracted by a working oil supplied from an oil hydraulic pump 71, the other end of the base 14 is pivoted to move vertically, so that the second mobile unit 16 is in contact with or spaced apart from the ground. In other words, when the underframe 10 stops in contact with the ground, the additional cylinder 17 is contracted to move up the other end of the base 14, so that the second mobile unit 16 is spaced apart from the ground. (In view of the base 14, the other end of the base 14 is pivoted, based on the one end fixed on the underframe 10 by a hinge.) Then, the first mobile unit 15 is always in contact with the ground, and the bottom of the underframe 10 does not permit rotation of the first 10 mobile unit 15 in contact with the ground. (Since a contact piece 19 in contact with the ground is formed on the bottom of the underframe 10, the underframe 10 is prevented from being supported by the first mobile unit 15 only.) To move the underframe 10 by being spaced apart from the ground through the first and second mobile units 15 and 16, the additional cylinder 17 is extended to move down the other end of the base 14 where the second mobile unit 16 is positioned, so that the one end of the base 14 moves up. Since the one end of the base 14 is fixed to the underframe 10, one side (a) of the underframe 10 at which the one end of the base 14 fixed moves up at the same time that the one end of the base 14 moves up. Subsequently, when the additional cylinder 17 is extended until the second mobile unit 16 is in contact with the ground, the other side (b) of the underframe 10 also moves up, based on the other end of the additional cylinder 17 fixed at the underframe 10 and the one end of the base 14. Accordingly, since the underframe 10 is spaced apart from the ground while the first and second mobile units 15 and 16 are in contact with the ground, the underframe 10 is moved through the first and second mobile units 15 and 16. (The base 14, first and second mobile units 15 and 16, and additional cylinder 17 may be combined in a single body in a hollow square pipe-shaped body with an open bottom, to be installed at the right/left and upper/lower sides of the underframe 10, depending on the user's exemplary embodiment.) Further, as illustrated in FIGS. 1 through 7, a foldable chair 80 and desk 90 for work are installed at one side on the top of the underframe 10. The chair 80 for work is attachable/detachable to/from the underframe 10. (A light emitting member, such as an LED stand, 82 may be installed on the desk 90 for work.) As illustrated in FIG. 9, the chair 80 for work comprises: a seat 81 on which a 11 user sits down; a backrest 82 connected to the seat 81 by a hinge and folded with the seat 81; and a support 83 positioned under the seat 81 and supporting the chair 80. A bent part 84 is formed at one end of the support 83 and a fixing member 85 is connected to the other end of the support 83 by a hinge. The bent part 84 is bent in a 'L' shape, toward the ground. The bent part 84 is fitted in a groove 15 formed to be depressed at a certain depth from the surface of the underframe 10. The groove 15 has to be in the shape to receive the bent part 84. The fixing member 85 connected to the other end of the support 83 by the hinge is pivoted and has a 'E 'shape forming a receiving part 86. As illustrated in FIG. 8, the side of the underframe 10 is received in the receiving part 86 of the fixing member 85, so that the chair 80 for work is secured to the underframe 10 by the bent part 84 and the fixing member 85. The chair 80 is attachable/detachable to/from the underframe 10 at the user's choice. As illustrated in FIGS. 4 through 7, the workbench 20 is horizontally positioned at the other side on an upperframe 40, so as to be opposite to the foldable chair 80 and table 90 for work. In the present invention, the mineral sample box (hereinafter, referred to as a 'workpiece') is placed on the workbench for detection, management, recordation and the like of the samples and materials contained in the box. A number of ball rollers 21 are rotatably installed and a part of each ball roller 21 is exposed on the workbench 20. The ball rollers 21 are equally spaced apart from one another. The workpiece placed on the workbench 20 is smoothly moved on the workbench 20 by the ball rollers 21. Further, fence members 22 are positioned so as to vertically stand at an edge of the workbench 20, to prevent the workpiece from falling off the workbench 20. The fence members 22 are usually positioned to stand vertically. However, when 12 the workpiece needs to be placed on the workbench 20, the fence members 22 are moved from a vertical position to a horizontal position so as to be level with the workbench 20 on the same plane. To this end, as illustrated in FIGS. 10 and 11, fixing shafts 26 are installed at the four sides of the workbench 20. A first combination member 27 with a tongue receiving opening 28 is positioned at the outer circumference of the fixing shaft 26. A supporting shaft 23 is extended from the fence member 22. The supporting shaft 23 is bent and extended toward the fixing shaft 26. A second combination member 24 is formed at the end of the supporting shaft 23. The second combination member 24 is rotatably connected at the outer circumference of the fixing shaft 26. An extension tongue 25 is formed at the second combination member 24, to be fitted into the tongue receiving opening 28. That is, when the first and second combination members 27 and 24 engage such that the tongue receiving opening 28 receives the extension tongue 25, combination members 27 and 24 are connected to each other, each fence member 22 vertically stands, preventing the workpiece placed in the workbench 20 from falling off the workbench 20. To place the workpiece within the fence members 22, each fence member 22 is moved from the vertical position to the horizontal position after moving each fence member 22 in the length direction of the fixing shaft 26 so that the extension tongue 25 of the second combination member 24 disengages the tongue receiving opening 28 of the first combination member 27. Further, to raise the workbench 20 in height, as illustrated in FIG. 2, 3 or 7, the upper frame 40 and height-control member 50 are further included. Like this constitution and as illustrated in FIGS. 1, 4, 5 and 6, the workbench 20 is raised by using a link member 30. The upperframe 40 in a flat shape with a certain thickness 13 is installed on the link member 30, so as to be parallel to the underframe 10. The upperframe 40 is moved up by the link member 30. The upperframe 40 is relatively small in dimensions, compared with the underframe 10. As illustrated in FIG. 2, the upperframe 40 may be in a similar size to that of the workbench 20, so that a worker can work, sifting down on the chair 80 for work installed on the underframe 10. Otherwise, as illustrated in FIG. 3, the upperframe 40 may have the size such that the worker can work on the workbench 20, standing on the upperframe 40. Further, after a plurality of height-control members 50 for controlling a length in a vertical direction are installed on the top of the upperframe 40 moved vertically in the above manner, the workbench 50 is fixedly installed on the height-control members 50. The height-control members 50 include a plurality of links connected by a joint or hinge, so that the length is controlled vertically and the length is fixed at a desired position. The height-control members 50 may be realized in various methods, according to the user's exemplary embodiments. As illustrated in FIGS. 1 through 7, the link member 30 is positioned between the underframe 10 and the workbench 20. More specifically, as illustrated in FIG. 5, a single link member 30 includes a plurality of links cross-connected to each other by a pin, to move the workbench 20 vertically. Otherwise, as illustrated in FIG. 6, a plurality of the link members 30, each link member 30 formed of a plurality of links cross-connected to each other by a pin as shown in FIG. 5, are connected to one another by using hinges at ends of each link member 30, so as to be extended in length, thereby increasing the height of the workbench 20 to be moved up. The vertical movement of the link member 30 is made by a cylinder 60, which will be described later. The link members 30 positioned to be opposite to each other may be installed in any direction, i.e., lengthwise (FIG. 5) or widthwise (FIG. 6), at the 14 user's choice. As illustrated in FIGS. 1 through 7, one end of each cylinder 60 is connected to the oil hydraulic pump 71 installed in the underframe 10 and the other end of the cylinder 60 is fixedly connected to the link member 30. As the other end of the cylinder 60 connected to the link member 30 is extended/contracted by the working oil supplied from the oil hydraulic pump 71, it moves the link member 30 vertically. If the cylinder 60 is capable of moving the link member 30 vertically, the position at which the cylinder 60 is installed may vary at the user's choice. As illustrated in FIGS. 1 through 7, the workbench control lever 70 is installed on the underframe 10. It is most preferable to install the workbench control lever 70 proximately to the chair 80, so as to be easily operated after the worker rides on the upperframe 40 or underframe 10 of the riding lift table according to the present invention. The workbench control lever 70 is connected to the oil hydraulic pump 71 supplying the working oil to the cylinder 60. (Oil hydraulic lines have to be constituted between the workbench control lever 70 and the oil hydraulic pump 71 and between the oil hydraulic pump 71 and the cylinder 60, using an oil hydraulic hose through which the working oil is movable and a driving motor for driving the oil hydraulic pump 71. The working oil, air or oil may be applied depending on whether the cylinder 60 and the additional cylinder 17 are oil-hydraulic or pneumatic.) In accordance with the present invention as described above, to do various work, such as detection, management, recordation and the like of the mineral sample boxes positioned and held, at different heights, on the multilevel rack, the worker rides on the lift table, moves the lift table close to the position of the rack where the mineral sample boxes are held, without any troubles, and performs the work, using the workbench, chair and shelves installed on the lift table. Therefore, 15 there is no need to lower the mineral sample boxes on the ground and to return them to their original positions after performing the work. Furthermore, in accordance with the present invention having the above-described constitution, the working efficiency is improved, the working time is shortened, the number of required workers is reduced and the stability and safety during the work is increased. While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 16