mixtures of augite, titaniferous iron, feldspar, and hornblende, may have been the same at different epochs, sometimes approximating more to basalt and sometimes to trachyte; and, (as we learn from the important researches of Mitscherlich, and the analogy of artificial igneous products) chemical substances may have united in definite proportions in a crystalline form: in all cases we recognise that substances similar in composition have arrived at the surface of the earth by very different ways; either simply upheaved, or penetrating through temporary fissures; and that breaking through the older rocks, (i. e. the earlier oxydized crust of the globe), they have finally issued as lava currents from conical mountains having a permanent crater. To confound together phenomena so different is to throw the geological study of volcanos and volcanic action back into the obscurity from which, by the aid of numerous comparative observations and researches, it has gradually began to emerge.
The question has often been propounded: What is it that burns in volcanos,—What produces the heat which melts and fuses together earths and metals? Modern chemical science has essayed to answer, that what burns are the earths, the metals, the alkalies themselves; viz. the metalloids of those substances. The solid and already-oxydised crust of the globe separates the surrounding atmosphere, with the oxygen which it contains, from the inflammable unoxydised substances in the interior of our planet: when those metalloids come in contact with the oxygen of the atmosphere there arises disengagement of heat. The great and celebrated chemist who propounded this explanation of vol-
