798 THE WONDERS OF GEOLOGY. Lect. VIII. 



25. Structure of Slate Rocks. — The fineness of 

 grain, general aspect, hardness, and texture of these rocks, 

 are too well known, from the universal employment of slate 

 for economical purposes, to require particular description. 

 The colour usually approaches to blue, grey, green, and a 

 dull purple ; and the texture is very fine, although occa- 

 sionally the slate passes into sandstone and greywacke. 

 The structure is laminated, and the planes of deposition are 

 commonly well marked ; but there are also divisional lines 

 called cleavage planes, which traverse the sedimentary, and 

 give to these ancient argillaceous rocks a very peculiar 

 character. 



Lign. 1S2. — Slate rocks at Whitesand bay, near St. David's, 

 Pembrokeshire. 



(The lines of cleavage and stratification coincident.)* 



In some instances, the lines of cleavage are in the same 

 plane as those of the strata, as in this section {Lign. 182) ; 

 but commonly the cleavage is in a different direction to the 

 stratification, the respective lines crossing each other at 

 various angles. In the quartzose grit and sandstone of 

 Llandovery {Lign. 183, fig. 1), and in the slate rocks at 

 Whitesand bay, in Pembrokeshire {fig. 2), the discrepancy 

 between the lines of deposition and of cleavage is strongly 

 marked. The beds termed greywacke contain fragments and 

 pebbles of clay-slate, and other rocks, and have evidently 

 been indurated by high temperature : in proportion as the 

 extraneous substances are large or small, abundant or 

 scanty, the compound rock is termed a greywacke-con- 



x Silurian System, p, 390. 



