METALLIFEROUS LODES, VEINS, MASSES. 175 



masses, like the dio rites and basa'lts, which also shows that the 

 matter came from below upwards, and that it was driven with 

 great force. These facts do not present themselves in a particular 

 locality only, but are observed in all parts of the world. 



The state of pasty fusion in which the granites were, is indicated by the 

 manner in which these rocks are enveloped in certain sedimentary deposits, 

 or effused on the different soils they pass through. In the coal-measures 

 of La Pleau, to the south-west of Ussel, a portion of the formation has been 

 enveloped by porphyroid granites, which are found above and below. The 

 coal is there hard, as on all the plateau, and the deposit is very irregular. 

 In a great many localities, we find granite superposed on all sedimentary 

 deposits from schists, and the most ancient rocks, to those of the jura'ssic 

 period. There are different places in the Alps, where one may touch at the 

 same time, superposed rocks of crystallization and the subjacent sedimentary 

 deposit. 



The action of granitic rocks on those through which they pass is the 

 same as that of the preceding rocks ; compact, o'olitic, and earthy lime- 

 stones are converted into saccharo'id limestones, from which organic re- 

 mains have most frequently disappeared ; they assume bright colours of 

 every kind, green, red, black, &c., and, in contact with mica, are filled with 

 garnets and various other crystalline substances. They are often converted 

 into dolomites, which are nowhere more abundant than in formations of 

 granite and sometimes into gypsum, as proved by the out-croppings of this 

 substance in certain parts of the Alps. Clays, and various arena'ceous sub- 

 stances are transformed into jasper, and finally assume the characters of 

 mica'ceous or talcose schist, and gneiss. Simple sandstones of sedimentary 

 formations, on the approach of granite, are converted into beds of granular 

 quartz. It sometimes happens that modified schistose sandstones still pre- 

 serve their arena'eeous structure, although they may have become very 

 solid ; even the mica-schists to which they pass contain here and there thin 

 strata of sandy quartz, interposed between laminae of mica, which seems to 

 announce the remains of ancient modified sandstone. 



Granitic rocks, referred to different ages, are very abundant on the sur- 

 face of the globe ; being found sometimes in very lofty mountain chains, 

 and sometimes forming rounded hills disintegrated on the surface, and cover- 

 ing considerable extents of country. 



21. Metalliferous lodes, veins, masses. The dolomisa'tion 

 and the sulphatisa'tion of limestones, the presence of various sub- 

 stances in adjacent rocks, are not the only facts referable to the 

 passage of igneous rocks from the bosom of the earth. It also 

 happens that, on the contact of the new with the ancient rock, the 

 deposits are filled with different metallic minerals, either dissemi- 

 nated or injected into fissures, and between beds, or accumulated 

 in small masses, sometimes united by slender threads. This has 

 been remarked by M. Dufrenoy in regard to iron ores in the Py- 

 renees, which are found either in limestone, or placed between 

 sedimentary deposits and the granite which upheaved the solid 

 mass. 



It is evident, lodes or seams of ores are related to igneous action. As to 

 those which are deposited in veins, it is to be remarked, we have never had 

 occasion to follow them to a sufficient depth to ascertain whether they ter- 



2 1 . How are metalli'ferous veins produced ? 

 ' 36 



