Ch. XXXVI.] CLEAVAGE OF CRYSTALLINE ROCKS. 755 



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 indisputably 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 having a more jas- 

 pery 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 character of the rock in the same planes." ? 

 As one step farther towards tracing a passage from planes of cleavage 

 to those of foliation, Professor 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. 



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

 homogeneous 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 countries 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 deposition, because the former were by far the most marked 

 of the two. Now if such slaty masses should become highly crys- 

 talline, and be converted into gneiss, hornblende-schist, or any other 

 member of the hypogene class, the cleavage planes might possibly re- 

 main more visible than those of stratification. Professor Henslow 

 had noticed, so long ago as the year 1821, that the lamination of the 

 chloritic and other crystalline schists in Anglesea was approximately 

 in the planes of bedding; and Professor Ramsay, in 1841, observed 

 the same in regard to the gneiss and mica-schist of Arran. The last- 

 cited geologist says, in reference to Anglesea, that the metamorphism 

 probably took place when the Lower Silurian volcanoes were in activ- 

 ity, and therefore long before the cleavage of the Welsh rocks ; for 

 the cleavage of the latter affects in common the Lower Silurian and 

 the Cambrian strata. In the same memoir he adds, when referring; to 

 Mr. Darwin's theory of foliation, "that if the rocks be uncleaved 

 when metamorphism occurs, the foliation planes will be apt to coin- 

 cide with those of bedding ; but if intense cleavage has preceded, then 

 we may expect that the planes of foliation will lie in the planes of 

 cleavage." J 



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



f Sedgwick, Geol. Trans., Second Series, vol. iii. p. 471. 



X GeoL Quart. Journ., 1853, vol. ix. p. 172. 



