458 BUTE. GEOLOGY. 



or pale grey; and, not uncommonly, these colours are 

 intermixed in the same specimen, with the more common 

 dark blue or pale blue tint so prevalent in this rock. 

 This variety is generally fissile, and either straight or mi- 

 nutely undulated : the beds in which it prevails are in many 

 places wrought for economical purposes, throughout the 

 whole extent of this series from Arran across the main- 

 land. Occasionally, it refuses to split into parallel plates, 

 the continuity of the laminae being interrupted by the 

 multiplicity of transverse joints; and it is often also 

 capable of being separated into thick pieces without being 

 subsequently divisible into thinner plates ; the laminar 

 continuity being in certain parts interrupted. The non- 

 fissile substance common in the clay slates, and analo- 

 gous to hone, or whet slate, also occurs in this series. 

 The finer variety occasionally becomes nearly black and 

 approaches to talcose schist, the laminse being separated 

 by that mineral ; and, still more rarely, it is found to 

 contain hornblende. In the progress of change, it also 

 becomes intermixed with distinct scales of mica, and 

 with quartz sand ; thus passing into graywacke. 



This latter substance presents a great number of va- 

 rieties ; graduating from the finest texture thus described, 

 through various stages, into a rock so coarse as to 

 deserve the title of a conglomerate. But the prevailing 

 varieties consist of sand or gravel united by clay slate \ 

 and like the micaceous schists before described, grains 

 of various sizes are rarely intermixed in the same fragment 

 or mass. Where they are of very large size, the rock 

 sometimes contains so little of the argillaceous cement 

 as to present ah almost entire congeries of gravel. Occa- 

 sionally, mica, chlorite, or fragments of the several schists 

 already described, are intermingled with these grains and 

 fragments. 



The limestone is generally in thin lamina?, interleaved 

 with the clay slate, which it peculiarly accompanies. It 

 is commonly of a fine texture with a smooth even frac- 



