Volume 2

Principles of geology : being an inquiry how far the former changes of the earth's surface are referable to causes now in operation. / By Charles Lyell ...

  • Lyell, Charles, Sir, 1797-1875.
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Principles of geology : being an inquiry how far the former changes of the earth's surface are referable to causes now in operation. / By Charles Lyell ... Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    of the mass of modern lavas. When it is in great excess, lavas are called trachytic; when augite (or pyroxene) predominates, they are termed basaltic. But lavas of an intermediate composition occur, which from their colour have been called gray-stones. The abundance of quartz, forming distinct crystals or con- cretions, characterizes the granitic and other ancient rocks, now generally considered by geologists as of igneous origin ; whereas that mineral is rarely exhibited in a separate form in recent lavas, although it enters largely into their composition. Hornblende, so common in hypogcne rocks, is rare in modern lava; nor does it enter largely into rocks of any age in which augite abounds. It should, however, be stated, that the experi- ments of M. Gustavus Rose have made it very question- able whether the minerals called hornblende and augite can be separated as distinct species, as their different varieties seem to pass into each other, whether we consider the characters derived from their angles of crystallization, their chemical composition, or their specific gravity. The difference in form of the two substances may be explained by the different circum- stances under which they have been produced; the form of hornblende being the result of slower cooling. Crystals of augite have been met with in the scoria; of furnaces, but never those of hornblende ; and crystals of augite have been obtained by melting hornblende in a platina crucible, but hornblende itself has not been formed artificially. * Mica occurs plentifully in some recent trachytes, but is rarely present where augite is in excess. * Bulletin de la Soc. G6ol. de France, tom. ii. p. 206.