564 



MR. G. W. LAMPLUGH ON THE 



[JNov. 1895, 



II. General Description oe the Crush-Conglomerates. 



Although the conglomeratic rocks to be described vary considerably 

 in different exposures, and even in different portions of the same 

 exposure, there are yet certain characteristics which are common to 

 the whole of them. Essentially, these crush-conglomerates are rocks 

 made up of scattered fragments set in a slaty matrix. 



The fragments vary in composition, but most frequently are com- 

 posed of fine-grained grit or sandstone, and of sandy slate ; with less 

 abundant pieces of coarsish grit, of banded flaggy slate, and of purely 

 argillaceous smooth blue slate or shale. These fragments are 

 usually phacoidal or lenticular in outline, 1 but the shape differs in 

 different sections, and also according to the size. In some localities 

 many of the smaller inclusions are quite rounded, while in others 

 they take the form of subangular or spindle-shaped strips (see 

 figs. 7 and 8, p. 572). The fragments are of all sizes, from blocks 

 several feet in diameter (see fig. 1) to particles of microscopic 



Tig. 1. — Crag at the southern edge of Gronk SumarTc, Sulby. 

 (Shoiving large block of coarsish-grained grit in crush-con- 

 glomerate.) 



W. 20. S 



E.20.N". 



A. Block of sheared, slightly felspathic grit, about 8 feet long, embedded with 



many fragments of fine-grained flagstone and hard clay-slate in a matrix 

 of bluish slaty material. 



B. Inclusions in which the original bedding is still visible. 



The arrow indicates the probable direction of the shearing. 



dimensions. They are, as a rule, indiscriminately mingled, large and 

 small, in a highly-sheared slaty matrix ; but the average size of the 

 inclusions varies greatly in different localities, and there is some- 

 times a total absence of the larger fragments. 



A glossy sericitic face, evidently the result of shearing, coats the 

 harder inclusions ; and planes of similar character are abundant 

 throughout the matrix, and sometimes cut the fragments. These 

 schistose planes determine the weathering and the fracture of the 



1 Such fragments often appear nearly circular when cut across at right angles 

 to their longer axis. 



