4 THEORY OF IGNEOUS ROCKS AND VOLCANOES. [I. 



Third Series, Vol. XXXIX. p. 52.) Besides, there are intru- 

 sive rocks, such as the phonolites, which are highly basic, and 

 yet contain but very small quantities of limp,, 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 hyperite, labradorite, diorite, and amphibolite, 

 which make so large a part of the Laurentian system in North 

 America, are of sedimentary origin. They are here interstrati- 

 fied with limestones, dolomites, serpentines, crystalline gneisses 

 and quartzites, which latter are often conglomerate. The same 

 thing is true of similar feldspathic rocks in the crystalline 

 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 ob- 

 servers to regard it as intrusive rock, although its general mode 

 of pccurrence 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 

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

 trate 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 intrusive granites, dolerites, euphotides, or serpentines 

 have an origin distinct from the metamorphic strata of the same 

 kind which make up such vast portions of the older stratified 

 formations ? 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 produced this plastic condition in various crystalline 

 rocks. Certain facts, such as the presence in them of graphite 



