306 ANNUAL OF SCIENTIFIC DISCOVERT. 



II. As the history of the crystalline rocks becomes better known, we find 

 that many which wore formerly regarded as exclusively of plutonic origin 

 are also represented among altered sedimentary strata. Crystalline aggre- 

 gates of quartz and feldspar with mica offer transitions from mica-schist, 

 through gneiss, to stratified granites, while the pyroxenic and hornblendic 

 rocks of the altered Silurian strata of Canada pass, by admixtures of 

 anorthic feldspars, into stratified diorites and greenstones. In like manner 

 the interstratitied serpentines of these regions are undoubtedly indigenous 

 rocks, resulting from the alteration of silico-magnesian sediments, although 

 the attitude of the serpentines in many countries has caused them to be 

 ranked, with granites and traps, as intrusive rocks. Even the crystalline 

 limestones of the Laurentian series, holding graphite and pyroxene, are 

 occasionally found enveloping broken beds of quartzite, or injected among 

 the fissures in adjacent silicious strata. From similar facts, observers in 

 other regions have been led to assign a plutonic origin to certain crystalline 

 limestones. "\Ve are thus brought to the conclusion that metamorphic 

 rocks, such as granite, diorite, dolerite, serpentine, and limestone, may, 

 under certain conditions, appear as intrusive rocks. The pasty or semi- 

 fluid state which these rocks must have assumed at the time of their dis- 

 placement, is illustrated by the observations of Daubre'e upon the swelling 

 up of glass and obsidian, and the development of crystals in their mass, 

 under the action of hep ted water, indicating a considerable degree of mobil- 



7 O O 



ity among the particles. The theory of igneo-aqueous fusion applied to 

 granites by Poulett Scrope, and Scheerer, and supported by Elie de Beau- 

 mont, and by the late microscopic observations of Sorby, should evidently 

 be extended to other intrusive rocks ; for AVC regard the latter as being in 

 all cases altered and displaced sediments. 



III. The silico-aluminous rocks of plutonic and volcanic origin are natu- 

 rally divided into two great groups. The one is represented by the granites, 

 ti'achytcs, and obsidians, and is distinguished by containing an excess of 

 silica, a predominance of potash, and only small portions of soda, lime, 

 magnesia, and oxide of iron. In the other group silica is less abundant, 

 and silicates of lime, magnesia, and iron predominate, together with 

 anorthic feldspars, containing soda, and but little potash. To account for 

 the existence of these two types of plutonic rocks, Prof. J. Phillips supposes 

 the fluid mass beneath the earth's crust to have spontaneously separated 

 into a lighter, silicious, and less fusible layer, overlying a stratum of denser 

 basic silicates. In this Avay he explains the origin of the supposed granitic 

 substratum, of the existence of which, however, the study of the oldest 

 rocks affords no evidence. From these two layers, occasionally modified 

 by admixtures, and by partial separation by crystallization and eliquation, 

 Prof. Phillips suggests that we may derive the different igneous rocks. 

 Bunsen and Durocher have adopted, with some modifications, this view; 

 and the former has even endeavored to calculate the composition of the 

 normal trachytic and pyroxenic magmas (as he designates the two sup- 

 posed zones of fluid matter underlying the earth's crust), and then seeks, 

 from the proportion of silica in any intermediate species of rock, to deduce 

 the quantities of alkalies, lime, magnesia, and iron, which this should 

 contain. 



So long as the trachytic rocks are composed essentially of orthoclase and 

 quartz, and the pyroxenic rocks of pyroxene and labradorite, or a feldspar 

 approaching it in composition, it is evident that the calculations of Bunseii 



