196 Igneous Rocks and Volcanos. 



on the volcanic rocks of Iceland, and extended by Streng to simi- 

 lar rocks in Hungary and Armenia. These investigators suppose 

 a trachytic and a pyroxenic magma of constant composition, re- 

 presenting 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 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 with the results of analysis. 

 (Streng, Annates 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 sili- 

 cates 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 field 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, crystalline schists and quartzites, which are 

 often conglomerate. The same thing is true of similar feldspathio 

 rocks in the altered Silurian strata of the Green Mountains. These 

 metamorphic strata have been exposed 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 sedimentary origin. We 

 find in the Laurentian system that the limestones sometimes 

 envelope the broken and contorted fragments of the beds of quart- 

 zite, 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 dis- 

 tinct from the metamorphic strata of the same kind, which make 

 up such vast portions of the older stratified formation 2 To sup- 

 pose that each of these sedimentary rocks has also its representa- 



