70 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



the structure is very far from uniform, and more analogous to that 

 of a concretionary than to that of a bedded rock. On the contrary, 

 in typical slates there is an almost complete uniformity in a hori- 

 zontal direction along each thin bed, though each may differ much 

 vertically from its neighbours in the relative amount of the different 

 constituents, even when the layers are as thin as paper. The whole 

 structure is, in fact, just such as would result from the depo- 

 sition of material sorted by gentle currents, and subsequently com- 

 pressed vertically by the pressure of overlying strata, or laterally by 

 that which gave rise to slaty cleavage. At the same time it appears 

 to me equally certain that laminar minerals of the chlorite or talc 

 groups have been formed since the deposition of the rock, and crys- 

 tallized in situ, without materially altering the general structure. 

 Some thin bands, and perhaps even thicker beds, are indeed exclu- 

 sively composed of these more recently formed minerals ; but the 

 development in situ of hydrous silicates is a very different thing from 

 the crystallization of the mica. For these reasons I am disposed to 

 think it far more probable that the principal, typical, micaceous 

 constituent of the slates now under consideration was derived from 

 the disintegration of an older rock, and that the difference between 

 what we may call kaolinitic and micaceous clay-slates depended on 

 the constitution of the rocks which furnished the material. Fels- 

 pathic felsites and coarse-grained granites, even when very mica- 

 ceous, could yield only kaolinitic clays ; whereas the fine-grained 

 micaceous felsites already mentioned could yield, and, as I think, 

 have yielded, the material of the micaceous clay-slates. In them 

 the mica exists in such small particles that it has not been sepa- 

 rated from the kaolin, whereas in the case of granites the plates of 

 mica, being comparatively very large, chiefly remained associated 

 with the quartz, and gave rise to micaceous sandstones. Perhaps 

 some minor difficulties may remain ; but this supposition will ex- 

 explain the principal facts in a very satisfactory manner. As an 

 excellent illustration of this subject, I will describe some of the 

 slate rocks of North Wales. 



Slate Rocks of North Wales, 



in order to learn the source of the material of the fine-grained 

 beds, it is obviously best to study such a coarse-grained deposit as 

 that met with in a small quarry at Felin Cochwillin, Bethesda, near 

 Bangor. In this rock the constituent fragments vary from -^ to 

 Y^ of an inch in diameter. I presume that most geologists would 

 call it an ash bed ; but at the same time I think that no simple 

 examination of the rock in its natural state would suffice to decide 

 whether it was a true ash or formed of the detritus of igneous 

 rocks. Careful microscopical examination does, however, show that 

 it was in all probability a true felsitic ash, since the grains are not 

 as though formed by the mere breaking up of a solid rock consoli- 

 dated under pressure. Some of the fragments are an imperfect 

 pumice ; and the crystals of felspar and the fragments of the felsitic 



