462 Cleavage and Foliation. part n. 



great strata of porphyritic conglomerate in central 

 Chile, and where there could be but little doubt about 

 the bedding, I observed similar slight differences in com- 

 position, and likewise some distinct thin layers of 

 epidote, parallel to the highly inclined cleavage of the 

 mass. Again, I incidentally noticed in North Wales, 1 

 where glaciers had passed over the truncated edges of 

 the highly inclined laminae of clay-slate, that the surf ace , 

 though smooth, was worn into small parallel undula- 

 tions, caused by the component laminae being of slightly 

 different degrees of hardness. With reference to the 

 slates of North Wales, Professor Sedgwick describes the 

 planes of cleavage, as ' coated over with chlorite and 

 semi-crystalline matter, which not only merely define the 

 planes in question, but strike in parallel flakes through 

 the whole mass of the rock.' 2 In some of those glossy 

 and hard varieties of clay-slate which may often be 

 seen passing into mica-schist, it has appeared to me that 

 the cleavage-planes were formed of excessively thin, 

 generally slightly convoluted, folia, composed of micro- 

 scopically minute scales of mica. From these several 

 facts, and more especially from the case of the clay- 

 slate in Tierra del Fuego, it must, I think, be con- 

 cluded, that the same power which has impressed on 

 the slate its fissile structure or cleavage has tended 

 to modify its mineralogical character in parallel 

 planes. 



Let us now turn to the foliation of the meta- 

 morphic schists, a subject which has been much less 

 attended to. As in the case of cleavage-laminae, the 

 folia preserve over very large areas a uniform strike : 

 thus Humboldt 3 found for a distance of 300 miles in 



1 « London Phil. Mag.' vol. xxi. p. 182. 



* * Geological Trans.' vol. iii. p. 471. 



* ' Personal Narrative,' vol. vi. p. 591, et seq. 



