89 



somewhat more sandy and micaceous than the blue ones. The discolouration 

 corresponds generally with the sm-face of the fragment, but is more spherical 

 towards the centre, and upon again splitting open one of the two pieces into 

 which the whole was divided, a nodule, corresponding in shape with the weather 

 stain, was exposed to view. 



Secondly, there are instances in which there is no apparent difference 

 whatever between the character of the nodule and the surrounding rock, and 

 yet its spheroidal structure is not less distinctly marked. 



At the southern extremity of the Longmynd range, in the construction 

 of the railway from Craven Arms to Bishop's Castle, a considerable cutting ex- 

 posed a most interesting section of the lower purple Wenlock beds. Some four 

 or five years have elapsed since then and the strata are to be seen at the present 

 time in a state of semi-decay. Before long they will have assumed the mono- 

 tonous slope of the usual railway embankment. In the meantime the pro- 

 jecting masses of shale present a very interesting object of study as regards their 

 structure. It may be observed that before entirely falling to pieces these blocks 

 of stone almost invariably become spheroidal. In some cases the original sharp 

 edges and angles of the rock are yet preserved, in others some portions have been 

 removed, while in others a perfect spheroid has been exposed to view ; the origin 

 of which may however be traced in other as yet undecayed blocks stretched out 

 as it were in fine lines on the surface of the stone. 



In every direction may be seen in this railway cutting the indication of a 

 nodular structure embedded in large Cubical masses of rock, sometimes of the 

 diameter of two or more feet, sometimes of as many inches. Like the statue 

 which the sculptor imagines to be imprisoned in the shapeless marble, and which 

 he hastens with skilful chisel to reveal, these spheroidal masses exist in the 

 primeval rock awaiting the inevitable hand of time to bring them to light. 



Now the question remains to be considered what is the cause of this 

 structure? It does not, as far as I am aware, exist where slaty cleavage is 

 very marked, but where the general character of the rocks is rhomboidal, there 

 it seems to abound. In many of the instances last referred to, the joints of the 

 stone are continued through the spheroids unaffected by their curved structure ; 

 in some cases this curved structure assumes the fonn of concentric layers, like 

 the coats of an onion, and yet the general stratification of the external rook is 

 continued through them when not interrupted by the lines of curvature. This 

 is seen in a very beautiful example on the "Wliitcliffe, near Ludlow. Here it 

 would seem that the position of the three more prominent spherical masses was 

 determined by the occurrence of the somewhat parallel faults which run 

 obliquely from the top of the right-hand side of the section to the bottom of 

 the left. 



Has this peculiar structure been the result of an accumulation of 

 calcareous matter at particular spots when the whole stratum was deposited, 

 or is it the result of subsequent action ? 



The occurrence occasionally of a hard calcareous kernel in the centre of a 



