GEOLOGY. 301 



An analogous view was suggested by Bunsen, in his researches on the vol- 

 canic rocks of Iceland, and extended by Streng to similar rocks in Hungary 

 and Armenia. These investigators suppose a trachytic and a pyroxenic mag- 

 ma of constant composition, representing respectively the two great divisions 

 of rocks which we have just distinguished; and have endeavored to calculate, 

 from the amount of silica in any intermediate variety, the proportions in 

 which these compounds must have been mingled to produce it, and conse- 

 quently the proportions of alumina, lime, magnesia, iron-oxide, and alkalies 

 which such a rock may be expected to contain. But the amounts thus cal- 

 culated, as may be seen from Dr. Strong's results, do not always correspond 

 with the results of analysis. Besides, there are varieties of intrusive rocks, 

 such as the phonolites, which are highly basic, and yet contain but very 

 small quantities of lime, magnesia, and iron oxide, being essentially silicates 

 of alumina and alkalies in part hydrated. 



We may here remark, that many of the so-called igneous rocks are often 

 of undoubted sedimentary origin. It will scarcely be questioned that this is 

 true of many granites, and it is certain that all the feldspathic rocks coming 

 under the categories of hypcrite, labradoritc, euphotide, dioritc, amphibolite, 

 which make such so large a part of the Laurentian system in Xortli Amer- 

 ica, arc of sedimentary origin. They arc here iuterstratitied with lime- 

 stones, dolomites, serpentines, crystalline schists and quartzitcs, which are 

 often conglomerate. The same thing is true of similar feldspathic rocks in 

 the altered Silurian strata of the Green Mountains. These metamorphic 

 strata have been expo.-ed to conditions which have rendered some of them 

 quasi-fluid or plastic. Thus, for example, crystalline limestone may be seen 

 in positions which have led many observers to regard it as intrusive rock, 

 although its general mode of occurrence leaves no doubt as to its sediment- 

 ary origin. We find in the Laurentian system that the limestones some- 

 times envelop the broken and contorted fragments of the beds of quartzitc, 

 with which they are often interstratified, and penetrate like a veritable trap 

 into fissures in the quartzite and gneiss. A rock of sedimentary origin may 

 then assume the conditions of a so-called igneous rock, and who shall say 

 that any of the intrusive granites, dolerites, euphotides, and serpentines, 

 have an origin distinct from the metamorphic strata of the same kind, 

 which make up such vast portions of the older stratified formation? To 

 suppose that each of these sedimentary rocks has also its representative 

 among the ejected products of the central fire, seems a hypothesis not only 

 unnecessary, but, when we consider their varying composition, untenable. 



We are next led to consider the nature of the agencies which have pro- 

 duced this plastic condition in various crystalline rocks. Certain facts, such 

 as the presence of graphite in contact with carbonate of lime, and oxide of 

 iron, not less than the presence of alkaliferous silicates, like the feldspars in 

 crystalline limestones, forbid us to admit, the ordinary notion of the inter- 

 vention of an intense heat, such as would produce an igneous fusion, and 

 lead us to consider the view first put forward by Poulctt Scropc, and since 

 ably advocated by Schecrer and by Elie de Beaumont, of the intervention of 

 water aided by fire, which they suppose ma}" communicate a plasticity to 

 rocks, at a temperature far below that required for their igneous fusion. 

 The presence of water in the lavas of modern volcanoes led Mr. Scrope to 

 speculate upon the effect which a small portion of this clement might exert, 

 at an elevated temperature and under pressure, in giving liquidity to masses 

 of rock, and he extended this idea from proper volcanic rocks to granites. * 



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