1859.] HUNT — CHElkllCAL GEOLOGY. 491 



degree of mobiKty among the particles. The theory of igneo- 

 aqueous fusion applied to granites by Poulett Scrope and Scheerer, 

 and supported by Elie de Beaumont and by the late microscopic 

 observations of Sorby, should evidently be extended to other intru- 

 sive rocks ; for we regard the latter as being in all cases altered and 

 displaced sediments. 



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

 are naturally divided into two great groups. The one is represented 

 by the granites, trachytes, 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 felspars, containing 

 soda and but httle 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, siliceous, and less fusible layer, overlying a stratum of 

 denser basic silicates. In this way 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 separa- 

 tion 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 endeavoured to calculate the composition of the normal 

 trachytic and pyroxenic magmat (as he designates the two supposed 

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

 from the proportion of silica in any intermediate 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 ortho- 

 clase and quartz, and the basic rocks of pyroxene and labradorite, 

 or a felspar approaching it in composition, it is evident that the 

 calculations of Bunsen will to a certarn extent hold good ; but in the 

 analyses, by Dr. Streng, of the volcanic rocks of Hungary and 

 Armenia, we often find that the actual proportions of alkalies, Hme, 

 and magnesia vary considerably from those deduced from calculation. 

 This w^ necessarily follow when felspars like albite or anorthite 

 replace the labradorite in pyroxenic rocks. The phonolites are 

 moreover highly basic rocks, which contain but very small amounts 

 of Hme, magnesia, or iron, being essentially mixtures of orthoclase 

 with hydrous silicates of alumina and alkalies. 



§ IV. In a recent inquiry into the probable chemical conditions of 

 a cooling globe like our earth, I have endeavoured to show that in the 

 primitive crust all the alkalies, lime, and magnesia must have existed 

 in combination with silica and alumina, forming a mixture which 

 perhaps resembled dolerites, while the very dense atmosphere would 

 contain, in the form of acid gases, aU the carbon, chlorine, and sul- 

 phur, with an excess of oxygen, nitrogen, and watery vapour. The 

 first action of a hot acid rain, falling upon the yet uncooled crust. 



