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Pant I. Srcr. iv. § 2.] SCHISTOSE STRUCTURE. 307 
easily. It is in these planes that the differences of mineral 
structure and composition are ranged. Layers of siliceous, argillaceous, 
and calcareous material alternate, each varying in porosity and 
capability of being changed by permeating water. We may, 
therefore, expect that unless the original stratified structure has 
been effaced or rendered inoperative by any other superinduced 
structure, it will guide the metamorphic action of underground water, 
and will remain more or less distinctly traceable, even after very 
considerable mineralogical transformations have taken place. Even 
without this guiding influence, superheated water can produce a 
schistose structure, as Danbrée’s experiments upon glass, above 
cited, have proved. | 
The stratified formations consist largely of silica, silicates of 
alumina, lime, magnesia, soda, potash, and iron oxides. ‘These mineral 
_ substances exist there as original ingredients, partly in recognizable 
worn crystals, but mainly-in a granular or amorphous condition, 
ready to be acted on by permeating water under the requisite 
conditions of temperature and pressure. We can understand that 
any re-combination and re-crystallization of the silicates will 
probably follow the laminz of deposit, and that in this way a crys- 
talline foliated structure may be developed. Round masses of 
granite erupted among Paleozoic rocks, instructive sections may 
be observed where a transition can be traced from ordinary unaltered 
sedimentary strata, such as sandstones, greywackes and shales 
containing fossils, into foliated crystalline rocks to which the names 
of mica-schist and gneiss may be applied. (Book IV., Part 
VIII.) Not only can the gradual change into a crystalline foliated 
structure be readily followed with the naked eye, but with the aid of 
the microscope the finer details of the alteration can be traced. 
Minute plates of some micaceous mineral and small concretions of 
quartz or felspar may be observed to have crystallized out of the 
surrounding amorphous sediment. ‘These can be seen gradually 
increasing in size and number until the rock assumes a thoroughly 
foliated structure and passes into a true schist. Yet even in such 
a schist traces of the original and durable water-worn quartz-granules 
may be detected.’ Foliation is thus a crystalline segregation of the 
mineral matter of a rock in certain dominant planes which are 
probably for the most part those of original stratification, but may 
in some cases be those of joints or of cleavage.? Mr. Sorby has 
recognized foliation in these three sets of planes even among the 
same rocks.® 
Scrope many years ago called attention to the analogy between 
the foliation of schists and the ribbanded or streaky structure of 
—trachyte, obsidian and other lavas.* ‘This analogy has even been 
1 Sorby, Q. J. Geol. Soc. xxxyvi. p. 82. 
2 Darwin, “ Geological Observations,” p. 162. Ramsay, “ Geology of North Wales,” 
in Memoirs of Geol. Survey, vol. iii. p. 182, . 
3 Op. cit. p. 84. 
4 “ Volcanoes,” pp. 140, 300. 
Dame 
