STRUCTURE OF GRANITE. 33 



and to show the actual particles of which it consists here drawn 

 greatly magnified and apart from each other ; and secondly (tig. 8), 

 we suppose the same mass of clay to have been compressed from one 

 end, and to have yielded somewhat above. Here all the particles are 

 seen to have shifted their positions, and to be extended at right angles 

 to the pressure. Formerly the rock would split in the direction in 

 which the particles were deposited, now it splits in the new direction 

 into which the particles have crystallised. In this country slates 

 occur throughout the Cambrian, in the Silurian, and in the Devonian 

 rocks ; and are found in Cornwall, Devon, and West Somerset, in 

 Wales, the Lake Country, Scotland, and Ireland ; but the best known 

 slates are from the Lower Cambrian, at Penrhyn, Llanberis, and 

 Ffestiniog. In other countries slates occur, of all geological ages. 

 They almost always form mountain-masses with well-defined and 

 peculiar contours. 



Mr. Darwin first drew attention to the way in which the rocks 

 showing cleavage lie parallel to those showing foliation, and drew 

 the conclusion that foliation is only an intense form of cleavage ; 

 or is due to the same cause when the forces producing it are more 

 powerful. There are some intermediate kinds of rocks ; and Pro- 

 fessor Sedgwick found slates 1 with the cleavage planes sometimes 

 coated over w r ith chlorite and semi-crystalline matter, which not 

 only defines these planes, but extends in parallel flakes through the 

 whole mass of the rock. And when it was found that the granite 

 masses which often form the central bosses or axes, as they are 

 called, of mountain chains, are parallel to the overlying metamorphic 

 rock, it began to be suspected that granite also might be a product 

 of metamorphism ; especially as it consists of the same minerals as 

 gneiss, and only differs in having them mixed confusedly together, 

 instead of being arranged in parallel layers. Moreover, there are 

 many intermediate varie- 

 ties of rock ; and granite 

 in large masses often 

 shows an approach to a 

 banded structure, owing 

 to the mode in which 

 its minerals are arranged. 

 It will readily be under- 

 stood that if the consti- 

 tuents of gneiss have 

 arranged themselves in Fi - 9- 



crystalline layers at right angles to a single plane of pressure, it will 

 follow that if the different crystals arrange themselves simultaneously 

 at right angles to two or three planes of pressure, their disposition in 

 parallel layers will be interfered with, and the materials must become 

 mixed more or less confusedly together. And this is exactly what 

 happens in the central axis of a mountain, where the pressure is more 



K tense than on the nanks, and comes from both sides of the fold. 

 Sedgwick, " Structure of Mineral Masses," Trans. GeoL Soc., 2d series, vol. 3. 

 VOL. I. C 



