ON THE THEORY OF IGNEOUS ROCKS AND VOLCANOS. 203 



denser stratum of the basic silicates, upon whicli a lighter and more 

 silicious portion floats like oil upon water, and that these two liquids, 

 occasionally more or less modified by a partial crystalization and 

 eliquation, or by a refusion, give rise to the principal varieties of silici- 

 ous and basic rocks^ while from the mingling of the two zones of 

 liquid matter, intermediate rocks are formed. (Phillips' Manual of 

 Geology, p. 556, and Durocher, Amiales des Mines, 1857, vol, 1, p. 

 217.) 



An analogous view was suggested by Bunsenin his researches on the 

 volcanic rocks of Iceland, and extended by Streng to similar rocks in 

 Hungary and Armenia. These investigators suppose a trachytic and 

 a pyroxenic magma 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 inter- 

 mediate variety, the proportions in which these compounds must have 

 been mingled to produce it, and consequently the proportions of alumina, 

 lime, magnesia, iron-oxyd and alkalies which such a rock may be 

 expected to contain. But the amounts thus calculated, as may be 

 seen from Dr. Streng' s results, do not always correspond vsdth the 

 results of analysis. (Streng, Annales de Chimie et de Physique, 3rd 

 series, vol. 39, p. 52.) 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 oxyd, 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 feld- 

 spathic rocks coming under the categories of hyperite, labradorite, 

 euphotide, diorite, amphibolite, which make such so large a part of the 

 Laurentian system in North America, are of sedimentary origin. They 

 are here interstratified with limestones, dolomites, serpentines, crystal- 

 line schists and quartzites, 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 ex- 

 posed to conditions which have rendered some of them quasi-fluid or 

 plastic. Thus for example, crystalline limestone may be seen in posi- 

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

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

 sedimentary origin. "We find in the Laurentian system that the lime- 

 stones sometimes envelope the broken and contorted fragments of the 

 beds of quartzite, with which they are often interstratified, and pene- 



