608 FOLIATION AND CLEAVAGE. [Ch. XXXVI. 



cleavage is the first effect ;" or, at any rate, that the crystalline force 

 may have been most energetic in the direction of cleavage. As bearing 

 on this view, he says, " I was particularly struck in the eastern parts of 

 Terra del Fuego with the fact that the fine laminae of clay-slate, where 

 they cut straight through the bands of stratification, and therefore indis- 

 putably true cleavage-planes, differ slightly from one another in their 

 grayish and greenish tints of color, as also in their compactness, 

 and in some laminae havirig a more jaspery appearance than others. 

 This fact shows that the same cause which has produced the highly 

 fissile structure has altered in a slight degree the mineralogical char- 

 acter of the rock in the same planes."* As one step farther towards 

 tracing a passage from planes of cleavage to those of foliation, Pro- 

 fessor Sedgwick observes that in North Wales the surfaces of slates 

 are sometimes coated over with chlorite, " the crystals of which 

 have not only defined the cleavage planes but struck through the 

 whole mass of the rock."f So also, says Mr. Darwin, in some places 

 in South America crystals of epidote and of mica coat the planes of 

 cleavage. 



Mr. D. Sharpe inferred from observations made by him in the High- 

 lands of Scotland, in 1851, that the foliation of the gneiss and mica-schist 

 are upon the whole parallel to one another, but have no connection with 

 any original planes of stratification ; and he also conceives that the planes 

 both of cleavage and foliation in the Grampians, and in the region of 

 Mont Blanc in Switzerland (which last he examined in 1854) are parts of 

 great curves or anticlinal axes of considerable regularity.! In like man- 

 ner in South America the cleavage planes of the clay-slate had been sus- 

 pected by Mr. Darwin, notwithstanding their varying and opposite dips, 

 to be parts of large curves or foldings, having their summits cut off and 

 worn down.§ 



There seems to be no difficulty in imagining that in rocks of homo- 

 geneous composition the foliation may take place along planes previously 

 caused by the elongation of the materials along the dip of the cleavage ; 

 for experienced geologists have been at a loss to decide in many coun- 

 tries which of two sets of divisional planes were referable to cleavage, 

 and which to stratification ; and after much doubt, have discovered 

 that they had at first mistaken the lines of cleavage for those of deposi- 

 tion, because the former were by far the most marked of the two. Now 

 if such slaty masses should become highly crystalline, and be converted 

 into gneiss, hornblende- schist, or any other member of the hypogene 

 class, the cleavage planes would be more likely to remain visible than 

 those of stratification. Professor Henslow had noticed, so long ago as 

 "lie year 1S21, that the lamination of the chloritic and other crystalline 



* Geol. Observ. on South America, p. 155. 



f Sedgwick, GeoL Trans. 2d eer. toL iii p. 471. 



% D. Sharpe, Phil. Trans. 1852, and GeoL Quart. Journ. No. 41, 1855. 



§ Darwin, S. America, p. 155. 



