754 FOLIATION OF CRYSTALLINE ROCKS. [Ch. XXXVI. 



direction as the strike of the slaty cleavage ; for the true strata always 

 dip at right angles to the axis of elevation, and are parallel to it in 

 their strike. No argument, therefore, can be drawn in favor of a com- 

 mon origin from uniformity of strike in the slaty and foliated rocks-; 

 for we require, in addition, coincidence of dip ; and such is the 

 variability of the dip both of the slates and folia as to render this kind 

 of proof very difficult to obtain. 



That the planes of foliation of the crystalline schists in Norway 

 accord very generally with those of original stratification is a con- 

 clusion long since espoused by Keilhau.* Numerous observations 

 made by Mr. David Forbes in the same country (the best probably 

 in Europe for studying such phenomena on a grand scale) confirm 

 Keilhau's opinion. In Scotland, also, Mr. D. Forbes has pointed out 

 a striking case where the foliation is identical with the lines of strati- 

 fication in rocks well seen near Crianlorich on the road to Tyndrum, 

 about 8 miles from Inverarnon in Perthshire. There is in that 

 locality a blue limestone foliated by the intercalation of small plates 

 of white mica, so that the rock is often scarcely distinguishable in 

 aspect from gneiss or mica-schist. The stratification is shown by the 

 large beds and colored bands of limestone all dipping, like the folia, 

 at an angle of 32 degrees N.E.f 



In stratified formations of every age we see layers of siliceous sand 

 with or without mica, alternating with clay, with fragments of shells 

 or corals, or with seams of vegetable matter, and we should expect the 

 mutual attraction of like particles to- favor the crystallization of the 

 quartz, or mica, or felspar, or carbonate of lime along the planes of 

 original deposition, rather than in planes placed at angles of 20 or 40 

 degrees to those of stratification. 



In Patagonia, a series of thin sedimentary layers of tuff were ob- 

 served by Mr. Darwin to have become porphyritic, first where least 

 altered, by a process of aggregation, small patches of clay appearing 

 to be shortened into almond-shaped concretions, which in those places 

 where they were more changed had become crystals of felspar, having 

 their longer axes parallel to each other. In other associated strata, 

 grains of quartz had in like manner aggregated into nodules of crys- 

 talline quartz.]; 



May we not, then, presume that in rocks where no cleavage has 

 intervened, foliation and the planes of stratification will usually co- 

 incide, as in all cases where cleavage happens (as in the writing-slates 

 of the Niesen on the Lake of Thun in Switzerland, containing fucoids) 

 to agree with the original planes of sedimentary deposition? Mr. 

 Darwin conceives that " foliation may be the extreme result of the 

 process of which cleavage is the first effect ; " or, at any rate, that the 



* Norske Mag. Naturvidsk., vol. i. p. 71. 



f Memoir read before the Geol. Soc. London, Jan. 31, 1855. 



X South America, p. 149. 



