006 FOLIATION OF CRYSTALLINE ROCKS. [Ch. XXXVl 



James Forbes has so ably explained, showing that it is due to the fissuring 

 of a viscous body in motion.^ 



Whatever be the cause, the result, observes Darwin, is well worthy the 

 attention of geologists ; for in a volcanic rock of the trachytic series in 

 Ascension layers are seen often of extreme tenuity, even as thin as hairs, 

 and of different colors, alternating again and again, some of them com- 

 posed of crystals of quartz and diopside (a kind of augite), others of 

 black augitic specks with granules of oxide of iron ; and lastly, others 

 of crystalline felspar. It is supposed in this case that the crystal- 

 lizing force acted more freely in the direction of the planes of cleav- 

 age, produced when the pasty mass was stretched, whether because 

 confined vapors were enabled to spread themselves through the minute 

 fissures, or because the ultimate molecules had more freedom of motion 

 along the planes of less tension, or for some other reasons not yet under- 

 stood. 



After studying, in 1835, the crystalline rocks of South America, Mr. 

 Darwin proposed the term foliation for the laminae or plates into which 

 gneiss, mica-schist, and other crystalline rocks are divided. Cleavage, 

 he observes, may be applied to those divisional planes which render a 

 rock fissile, although it may appear to the eye quite or nearly homo- 

 geneous. Foliation may be used for those alternating layers or plates of 

 different mineralogical nature of which gneiss and other metamorphic 

 schists are composed. The cleavage planes of the clay-slate in Terra del 

 Fuego and Chili preserve a uniform strike for hundreds of miles in regions 

 where these planes are quite distinct from stratification. In the same 

 country the planes of foliation of the mica-schist and gneiss are parallel 

 to the cleavage of the clay-slate. Hence, we are tempted, at first sight, 

 to infer that some common cause or process, and that cause not con- 

 nected with sedimentary deposition, has impressed cleavage on the one 

 set of rocks and foliation on the other. But such an inference can only 

 be legitimately drawn in those rare cases where we are able, by a con- 

 tinuous section, to prove that not only the strike, but the dip of the slaty 

 cleavage on the one hand, and of the foliation on the other, precisely 

 coincide ; the cleavage at the same time not being parallel to the strati- 

 fication in the slate rock. In some examples cited by Mr. Darwin, in 

 Terra del Fuego, the Chonc:- Islands, and La Plata, this uniformity of dip 

 seems to have been traced in a manner as satisfactory as the nature of 

 such evidence will allow. But we must be on our guard against a 

 source of deception which may mislead us in this chain of i-easoning. 

 We are informed that in South America, as in other countries, the strike 

 of the cleavage in clay-slate conforms to the axis of elevation of the rocks 

 in the same districts. Hence it must follow that the folia of gneiss, 

 mica-schist, limestone, and other crystalline rocks, even if they strictly 

 coincide wath the planes of original stratification, will run in the same 

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



* Darwin, Volcanic Islands, pp. 69, 70. 



