JOINTS OF THE SILURIAN ROCKS. 



245 



assisted in their labours than even by the division of the mass into beds, for as 

 the master joints extend to great depths, often traversing all the strata in straight 

 and well-determined lines, they are not only the greatest aids in the extraction of 

 blocks of stone, but also the surest guides for the line of work. In fact, if a sufficient 

 number of joints cross each other diagonally to the strike, and at right angles to each 

 other and to the laminae of deposit, it is manifest, that the mass of rock so affected is 

 split into symmetrical blocks; and as the faces of the joints are smoother and more 

 regular than the surfaces of the beds, they are ready-made cleavages which help the 

 quarryman's labours. The joints of the Silurian rocks are, for the most part, straight- 

 cut chinks, either slightly open, or filled with a little shale. Wherever sufficiently open 

 to be pervious to water, (as is frequently the case,) they present weathered faces even 

 when first cut into in a quarry, and hence also it is, that the small concretions exposed 

 on their surfaces, are constantly found to present certain lines of swallow holes, pre- 

 viously mentioned as common in the vertical faces of the Ludlow rocks, the less co- 

 herent concretions having disintegrated, even in subterranean positions, by the access 

 of water, which has percolated for ages through these natural channels. 



Although the time has probably not yet arrived when we can distinctly assign a 

 cause for the formation of such joints, we may in the first instance endeavour to define 

 the epoch of their formation by reference to other phenomena. It has been pointed 

 out, that the Upper Silurian rocks, though in many parts deposited in straight bedded 

 layers, composed of varied materials, are also very prone to concretionary structure. 

 That structure was also shown to interfere abruptly with the laminae of deposit, 

 and so to truncate them, that there can be no doubt, the concretions were formed by 

 some chemical or electric action after the first aggregation of the surrounding strata. 

 In this way, therefore, we obtain evidence of two distinct successive operations as ex- 

 pressed in this wood-cut. 



First the successive deposition of the materials composing the beds 1, 1 ; secondly, the 

 arrangement of these materials so as to occasion particles of similar matter (limestone 

 for example, 2, 2,) to unite and form concretions distinct from the other laminae of de- 

 posit. But besides the beds and concretions, the joints of which we are now speaking, 

 are proofs of a third modification in structure, subsequent to both the others. This is 

 established by the fact, that the joints 3, 3 not only cut through the beds 1, 1, but also 

 through the concretions 2, 2. This is well seen in many deep quarries of the Wenlock 

 limestone, particularly at Lincoln Hill, near the Iron Bridge, where the joints descend 



3 



33. 



2 H 



