Page:The American Cyclopædia (1879) Volume XVI.djvu/421

 VOLCANO 401 nificant part of volcanic ejections, and it is doubtful whether the luminous appearances accompanying eruptions, which have given rise to the popular name of burning mountains, are dependent in any degree on combustion. They are most probably due solely to the intense ignition of the ejected matters. How far the movements of the lavas in the cra- ters of volcanoes are dependent on local and external conditions, and how far on deep- seated and occult agencies, is a question. It is by some supposed that the atmospheric wa- ters falling on a volcanic region, and sinking through the soil under the pressure of the col- umn of water above, may penetrate the lavas and become an efficient agent in their eleva- tion in the manner already pointed out; but there is good reason to believe that the force is in many cases far more deeply seated. The nature of the earthy materials ejected from volcanoes, and their relations to the other rocky matters of the earth's crust, next demand our notice. Setting aside the volcanic ashes and tufas, in which the constituent min- eral elements are much disguised, the solidi- fied lavas are conveniently grouped in the two classes of feldspathic or trachytic and au- gitic or doleritic rocks, according as the min- erals feldspar* and augite predominate in them. The rocks of the former class are character- ized by a comparatively low specific gravity, ordinarily from 2'4 to 2'7, a generally light color, a predominance of silica and alkalies,- and a scarcity of iron oxide, lime, and magne- sia ; while the augitic rocks have a specific gravity of from 2'8 to 3'2, are dark-colored, and contain in abundance the three bases last named. The amount of silica in the latter is generally from 40 to 50 per cent., while in the trachytic rocks it ordinarily ranges from 60 to 70 per cent. The analogies of the feldspathic class are with granitic rocks, into which the trachytes pass by insensible gradations, and they contain as essential elements feldspars allied to orthoclase, sanidine, and albite, some- times with a little hornblende or mica, and often with an excess of silica over that re- quired to form these minerals, which, instead of being separate in the form of visible quartz as in granite, is intimately blended with the base of the rock. These trachytic rocks are either coarsely crystalline, granular, and fri- able, or else fine-grained and compact with enclosed feldspar crystals. Dolerite, which may be taken as the most common form of the augitic rocks, consists of a mixture of au- gite and labradorite, generally with more or less disseminated magnetic or titanic iron. These rocks sometimes occur coarsely granular and crystalline, and at other times fine-grained or compact, constituting basalt. Other feldspars or related minerals, very distinct from those of the trachytic rocks, sometimes replace the labradorite, giving rise to various augitic rocks closely related to dolerite. Two of the most noticeable of these contain the minerals leucite and nepheline in place of labradorite. Olivine or chrysolite, moreover, often forms an impor- tant element in these augitic rocks. As all of these minerals, augite included, contain a com- paratively large proportion of basic elements, the rocks of this group are often designated " ba- sic " rocks, in contradistinction to which those of the trachytic group, in which silica or silicic acid predominates, are spoken of as " acidic " rocks. The results of the subsequent action of water upon many of these rocks are supposed to have given rise to certain modifications in composition since their ejection. In this way the cells and interstices in the porous lavas have been filled with various hydrated min- erals, such as zeolites, calcite, and chlorite, the results of a partial decomposition of the origi- nal mineral species. These two great groups and their subordinate varieties may be said to include all the volcanic rocks of both ancient and modern times. Mention should here be made of the vitreous variety known as ob- sidian, which is a glass generally formed from a trachytic lava, while pumice is a highly in- flated or vesicular form of the same. The difference between the crystalline and the vitreous state of rocks is due to the rate of cooling from fusion. The slags resulting from the fusion of the copper slates of Mansfeld in Germany, which closely resemble in composi- tion some doleritic rocks, form a glass when rapidly cooled, and a similar glass may be pro- duced from the melting of basalts ; but when slowly cooled the one and the other assume a crystalline structure. It was formerly sup- posed that the volcanic rocks of the present day and of the later geological periods differ- ed widely from those of more ancient times; but the results of careful microscopical and chemical study during the past few years go far to show that the constitution of these rocks, even as far back as palaeozoic times, is identical with that of recent ones, so that the volcanic activities which in former periods ex- hibited themselves over different regions of the earth's surface must have presented con- ditions similar to those of our own time. Al- though erosion has in most cases effaced the cones of the more ancient volcanoes, yet dikes and sheets of lava and beds of volcanic tufa still remain from remote geological periods. It would appear that some of these former outbursts of igneous rocks were on a grander scale than anything known in the historic pe- riod, and differed somewhat in the mode of their eruption. Thus at the beginning of the Cambrian period were poured out the enor- mous floods of doleritic lavas which, with vol- canic tufas and interstratified sandstones, form the copper-bearing or Keweenaw series of Lake Superior, which have a thickness of many thousand feet, and probably once occu- pied the entire breadth of the lake. Great outbursts both of trachytic and doleritic rocks occurred in the palaeozoic in the province of Quebec; and later, in the mesozoic period,