Projection to Geological Plans and Sections. 281 



Fig. 3, Plate XIII., is a ground plan of this district : that portion of it 

 which is unshaded represents the surface, with the houses, roads, fences, 

 &c. Those parts which are shaded are supposed to be excavated or re- 

 moved in order to afford a view of the strata, as, in fact, every section 

 requires such an assumed removal of the intervening objects. The strong 

 dotted lines denote the course of several sections required to illustrate 

 the subterranean formation and mining operations, and this, by the ordinary 

 mode, could only be done by 5 or 6 separate and detached sections, in 

 addition to the ground plan. By Isometrical Projection all these sections, 

 as well as the ground plan, are combined in one drawing, as in Plate XII* 

 every part of which admits of accurate measurement, and which conveys, 

 at a single glance, a clear idea of the connection of the various parts of the 

 district. It is obvious also that many more particulars might be introduced, 

 and that pictorial effect might be employed to give additional force and truth 

 to the representation. By this means, both the horizontal and vertical work- 

 ings may be delineated together. The connection of levels of different ran- 

 doms,* of shafts in various positions, the throw and hade of veins, &c, may 

 all be clearly represented, and form a faithful geometrical delineation of the 

 interior of mines, and of the geology of the district in which they are situated. 

 On the Ground Plan of Holyfield Mine (Fig. 2, Plate XIII.), the workings, 

 &c, are represented as they would appear if viewed from an extreme perpen- 

 dicular height, and are consequently projected on a horizontal plane, or base. 

 On this base, lines may be supposed to be drawn, forming geometrical 

 squares, as at a a, in the same figure, for the purpose of reference. In Isome- 

 trical Projection these squares assume the figure represented in Fig. 4, Plate 

 XIII., and that portion of the Sun Vein Level which is represented on the 

 horizontal squares from b to c is delineated on the Isometrical Squares in 

 Fig. 4 at d e. By means of the Isometrical Squares, any plan may be 

 represented with great ease and accuracy, and thus we obtain the ground 

 plan drawn in Isometrical Projection, But now arises the distinguishing 

 feature of this kind of projection ; the shaft, which on a common ground 

 plan is only shown by its circumference, as at Fig. 5, can be deli- 



* A mining phrase for the situation in which levels are driven in regard to height; thus, 

 a level above another is said to be driven at a higher random. 



