Page:The American Cyclopædia (1879) Volume XVI.djvu/422

 402 VOLCANO were erupted the immense masses of doleritic rocks which along the bay of Fundy, in the valley of the Connecticut, and from the Pali- sades of the Hudson S. W. through Pennsyl- vania, are associated with the red sandstones of that period. In the later tertiary time oc- curred the enormous eruptions which, extend- ing from northern California and Nevada to British Columbia, cover an area of over 200,- 000 sq. to. with an average thickness of 2,000 ft., and in one place with a thickness of 4,000 ft., consisting of alternate layers of trachytic and doleritic rocks. These great outflows of volcanic material from the lower Cambrian time were on a far more extended scale than any modern eruptions, and probably issued from vast fissures in the earth, due to widespread and deep-seated forces connected with the fold- ing of strata and continental movements. The hypotheses which have been proposed to ex- plain the origin of volcanoes are manifold, but for the most part can be briefly discussed. That of Davy and Daubeny was based upon the assumption that the earth's interior holds in an unoxidized condition silicon and the metal- lic bases of the earths and alkalies, which if brought in contact with the water of the ocean would react violently with it, generating a great amount of heat and giving rise to the elements of the silicated minerals which make up the vol- canic rocks, at the same time liberating in gase- ous compounds the chlorine and the sulphur of the sea salts, together with free hydrogen. These, with aqueous vapors, by their elasticity would rend the earth's crust, and thus account for the mechanical phenomena of earthquakes and volcanoes. This once celebrated hypothe- sis is now generally rejected as altogether base- less. Others, while rejecting the theory of an unoxidized nucleus, have sought to explain the origin of the volcanic heat by chemical reac- tions set up in the sedimentary deposits ; a view which the study of the thermal relations of chemical change shows to be untenable. Most geologists, adopting the view that the earth was originally in a fused condition, and that the interior is still intensely heated, and probably consists, at least to a great depth, of oxidized mineral matters not very unlike in composition to those at the surface, have sought therein an explanation of volcanic phe- nomena, though from this point there are several diverging hypotheses. Some have sup- posed that these materials immediately be- neath a crust of no great thickness still remain in a state of igneous fusion, and are arranged in two layers, the lighter of feldspathic mat- ter corresponding to the trachytes, and the heavier and inferior layer having the composi- tion of the dolerites. From these two liquid strata it is imagined that the various volcanic rocks are ejected by movements in the earth's crust, allowing the escape from time to time of portions of the one or the other, or of ad- mixtures of the two; while the intervention of sea water, as already explained, was also possible, giving rise to gaseous products and producing greater or less modifications in the composition of the rocks erupted. Such, in general terms, is the hypothesis adopted by a great many modern geologists, whether they admit the internal fluidity of the earth as a whole, or suppose it, though solid from the centre, to include beneath its surface still un- solidified lakes of molten rock, which may un- derlie the present volcanic regions. But the supposed separation of the cooling globe into two layers is a gratuitous hypothesis, and there are many chemical reasons for supposing that the upper layer of the earth's surface must have originally had a constitution not far removed from that of the dolerites, and more- over that the materials of the granitic and trachytic rocks were derived from the action of water upon this surface, and are not to be expected among the results of a simple igneous fusion. (See GRANITE.) It is also shown that certain anomalous types of eruptive rocks, which exhibit in their composition pretty wide divergences from both of the types above mentioned, are met with among water-formed rocks, and are doubtless of aqueous origin, as are also some augitio rocks. Another hypoth- esis in accordance with these facts has there- fore been advanced, which is tlrat the source of the volcanic and eruptive rocks of all ages is to be sought in the softening and melting of portions of the solid crust, including both the primitive doleritio layer and the various results of its alteration by aqueous agencies, embracing a great variety of rooks of sedi- mentary origin. In fact, we find among aque- ous crystalline rocks the two types already described as characterizing the igneous masses, and these types have been shown to be the natural results of chemical and mechanical forces always at work at the surface of the latter. The fusion of these and of the various heterogeneous materials which make up the sedimentary deposits, in the presence of the water and saline matters with which the rocks are always impregnated, would seem to ex- plain all the chemical phenomena of volca- nicity. The heat necessary to produce this result has been sought for either entirely in that transmitted from the earth's interior to the deeply buried portions of the crust, or in that mechanically evolved by the crushing of the deeply buried strata during the contrac- tion of the earth's crust and the consequent conversion of motion into heat. In reality the two influences must concur in producing this result. In this connection it has been pointed out that the great volcanic regions of modern times, wherever circumstances permit us to determine their geological relations, ap- pear to be those in which have occurred both great deposition of sediments burying to con- siderable depths the older rocks, and great movements of the earth's crust in comparative- ly recent geological periods. See Von Buch, Phyaikaluche Beschrtibung der Canaruclwr