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Updated: June 1, 2025
In the ninety-yard dike several large, apparently now quite isolated, fragments of mica-slate were embedded: but as their foliation was exactly parallel to that of the surrounding solid rock, no doubt these new separate fragments originally formed wedge-shaped depending portions of a continuous vault or crust, once extending over the dike, but since worn down and denuded.
Foliation of intrusive rocks. Relation of cleavage and foliation to the lines of tension during metamorphosis.
Near Monte Video, where the stratification, as it would be called, of the metamorphic series is, in most parts, particularly well developed, being as usual, parallel to the foliation, we have seen that a mass of chloritic schist, netted with quartz-veins, is entangled in gneiss, in such a manner as to show that it had certainly originated in some process of segregation: again, in another spot, the gneiss tended to pass into hornblendic schist by alternating with layers of quartz; but these layers of quartz almost certainly had never been separately deposited, for they were absolutely continuous with the numerous intersecting veins of quartz.
Hence the area, over which the cleavage of the slate and the foliation of the metamorphic schists extends with an average W.N.W. and E.S.E. strike, is about forty miles in a north and south line, and ninety miles in an east and west line.
Here is this earth, a marvellous onion, a series of encapsuled worlds, each successive foliation preserving the intimate secrets of its own irrecoverable life. And Man the Baby, neglecting the wonderful Earth he crawls on, has cried for the barren Moon! All science has begun with the stars, and Early Man seemed to himself merely the by-play of a great cosmic process.
Epidote likewise occurs in the gneiss in thin layers, parallel to the foliation of the mass. One large vein of a coarse granitic character was remarkable from in one part quite changing its character, and insensibly passing into a blackish porphyry, including acicular crystals of glassy feldspar and of hornblende: I have never seen any other such case.
These spheres are disposed in layers parallel to the foliation of the gneiss: and are the result of a metamorphic action of great intensity, effecting a complete rearrangement and crystallization of the quartz and mica in parallel planes, whilst the felspar is aggregated in spheres; just as in the rearrangement of the mineral constituents of mica-schists, the alumina is crystallized in the garnets, and in the clay-slates the iron into pyrites.
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.
From Khersiong to Dorjiling, gneiss is the only rock, and is often decomposed into clay-beds, 20 feet deep, in which the narrow, often zigzag folia of quartz remain quite entire and undisturbed, whilst every trace of the foliation of the softer mineral is lost.
The strain to which the rock has been subjected has resulted in a very fine lamination, and it is considerably weathered. 9 Calcarous Sericite Schist. Seven Miles East of Seal Lake. A dark compact rock, in which calcite and sericite predominate. Quartz is less plentiful. The results of shearing and pressure are very prominent and bring out the foliation, even in the calcite.
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