

Ch. XXXVI.] FOLIATION OF CRYSTALLINE ROCKS. 753 



ored to explain by referring to the Assuring 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 composed 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 crystallizing force acted more freely in the direction of the planes 

 of cleavage, 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 free- 

 dom of motion along the planes of less tension, or for some other rea- 

 sons not yet understood. 



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 

 homogeneous. 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 hun- 

 dreds 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 pro- 

 cess, and that cause not connected 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 continuous 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 stratification in the slate rock. In 

 some examples cited by Mr. Darwin, in Terra del Fuego, the Chonos 

 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 reasoning. We are informed 

 that in South America, as in other countries, the strike of the cleav- 

 age 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 with the planes of original stratification, will run in the same 



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

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