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Werner had found the granites, limestones, sandstones, schists, etc., occurring in a certain relative order in his native country; and he drew the very remarkable conclusion that this was the normal order in which these various rocks would invariably be found in all parts of the world, on the theory that this was the order in which these different rocks had been formed in the beginning, great layers of these different rocks having originally been spread completely around the globe one outside another like the coats of an onion.
In this respect the trachytes differ entirely from the members of the Basaltic family, and are more nearly allied to the granites. Obsidian, Pitchstone, and Pearlstone are only different forms of a volcanic glass produced by the fusion of trachytic rocks. The distinction between them is caused by different rates of cooling from the melted state, as has been proved by experiment.
These rocks are penetrated by numerous basaltic dykes. Assuming Sir A. Geikie's view to be correct, it is possible that we may have in the granite and quartz-porphyries of Mourne and Carlingford representatives of the granites, granophyres, and other acid rocks of the later period of Mull.
Sedimentary strata have been carried down, by movements of the earth's crust, far below the surface, covered by other deposits, and subjected to great heat, which, aided by the water contained in the rocks and various chemical reactions, has effected a re-arrangement of the mineral contents of the strata, so that by molecular movements, the metamorphic crystalline rocks, including interstratified granites and greenstones, have been formed.
The preceding chapter dealt more especially with prospecting as carried on in alluvial fields. I shall now treat of preliminary mining on lodes or "reefs." As has already been stated, the likeliest localities for the occurrence of metalliferous deposits are at or near the junction of the older sedimentary formations with the igneous or intrusive rocks, such as granites, diorites, etc.
Roughly marking out an area, to establish our prior claim to the ground amongst those already on the field, we returned to camp and gave Jim, who had been packing water from the granites, the joyful news.
Without being an expert in granites, I have seen enough of the granite works at home to feel quite sure that a moderate and judiciously managed investment here ought to return a handsome result.
Under the resistless pressure of crustal movements almost any rocks, sandstones, shales, lavas of all kinds, granites, diorites, and gabbros may be metamorphosed into schists by crushing and shearing. Limestones, however, are metamorphosed by pressure into marble, the grains of carbonate of lime recrystallizing freely to interlocking crystals of calcite.
It is bounded on the north by the granites of the Upper Palatinate; and on the south by Alpine limestone, transition-thonschiefer, and the mica-slates of the Tyrol. We arrived, on the 23rd of July, at the town of Nueva Barcelona, less fatigued by the heat of the Llanos, to which we had been long accustomed, than annoyed by the winds of sand which occasion painful chaps in the skin.
We saw in the mountains of Upper Orinoco, or of Parime, only granular granites containing a little hornblende, granites passing into gneiss, and schistoid hornblendes. Has nature repeated on the east of Esmeralda, between the sources of the Carony, the Essequibo, the Orinoco, and the Rio Branco, the transition-formation of Tucutunemo reposing on mica-schist?
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