THE FOUNDATION-STONES OF THE EARTH. 655 



hibit plienomena resemloling the effects of a tensile stress. Ma- 

 terials of a like character assume a more or less linear arrange- 

 ment ; the rock becomes slightly banded, and exhibits, as has 

 been said, a kind of flexion structure. The mass gradually as- 

 sumes a fragmental condition under the pressure, and its parti- 

 cles, as they shear and slide under the effects of thrust, behave to 

 some extent like those of a non-uniform mass of rock in a plastic 

 condition, as, for example, a slaggy glass. Illustrations of the 

 effects of direct crushing in a granitoid rock are common in the 

 Alps. Those of a shearing crush are magnificently developed near 

 the great overthrust faults in the northwest Highlands of Scotland. 

 It seems, then, to be demonstrated that by mechanical deformation, 

 accompanied or followed by molecular rearrangement, foliated 

 rocks, such as certain gneisses and certain schists, can be pro- 

 duced from rocks originally crystalline. But obviously there are 

 limits to the amount of change. To get certain results you must 

 have begun with rocks of a certain character. Hitherto we have 

 been dealing with rocks which were approximately uniform in 

 character, though composed of diverse materials — that is, with 

 rocks more or less granular in character. Suppose, now, the 

 original rock to have already acquired a definite structure — sup- 

 pose it had assumed, never mind how, a distinct mineral banding, 

 the layers varying in thickness from a small fraction of an inch 

 upward. Would this structure survive the mechanical deforma- 

 tion ? I can give an answer which will at any rate carry us a cer- 

 tain way. I can prove that subsequent pressure has frequently 

 failed to obliterate an earlier banded structure. In such a district 

 as the Alps we commonly find banded gneisses and banded schists 

 which have been exposed to great pressure. Exactly as in the 

 former case, the new divisional planes are indicated by a coating 

 of films of mica, by which the fissility of the rock in this direction 

 is increased. The mass has assumed a cleavage-foliation. I give 

 it this name because it is due to the same cause as ordinary cleav- 

 age, but is accompanied by mineral change along the planes of 

 division, while I term the older structure stratification-foliation, 

 because so frequently, if it has not been determined by a stratifica- 

 tion of the original constituents, it is at any rate a most extraor- 

 dinary imitation of such an arrangement. In many cases the new 

 structure is parallel with the old ; but in others, as in the " strain- 

 slip " cleavage of a phyllite, the newer can be seen distinctly cut- 

 ting across the older mineral banding. To put it briefly, I assert, 

 as the result of examining numbers of specimens, that, though in 

 certain cases the new structure is dominant, a practiced eye sel- 

 dom fails to detect traces of the older foliation, while in a large 

 number of instances it is still as definite as the stripe in a slate. 

 We have got, then, thus far — that pressure acting on rocks pre- 



