1870.] MACFARLANL — ON CRYSTALLINE ROCKS, 305 



not so much as aggregates of minerals, as mixtures of their 

 eiiemical components, alkaline and earthy silicates, which, during 

 crystallisation, arianged themselves into compounds of more 

 definite atomic composition, namely, into minerals. 



As has been already remarked, the primary source of all 

 original rocks must have been the original fluid globe, and also 

 that part of it, which, until the present day, has remained in a 

 state of igneous fluidity. The elements which originally com- 

 posed the fluid-globe must have been the same as those which 

 enter into the composition of the earth at the present day. If, 

 however, we leave out of consideration those volatile and craseous 

 elements which, from their nature, must have gone to form the 

 primitive atmosphere, and also the greater bulk of the metals, 

 which, from their gravity, must have accumulated at the centre 

 of the earth, we have the following list of substances, which in 

 all likelihood, constituted the upper zone of the original fluid- 

 globe: — Silicic, boracic, phosphoric, stannic, titanic, niobic, tung- 

 stic, and tantalic acids: among bases, potash, soda, lithia, lime, 

 magnesia, alumina, ferric oxide, zirconia, manganic oxide, mau- 

 ganous oxide, ferrous oxide, glucina, ceria, yttia, oxides of zinc, 

 lanthanum and uranium. All of these substances make their 

 appearance in original rocks, many of them however in compara- 

 tively minute quantity and entering only into the composition of 

 their so-called accessorial constituents. If we, for the sake of 

 clearness, lea\e these rarer substances aside for the present, we 

 have the followinir. which may be reuarded as the essential 

 chemical constituents of original rocks: 



Silicic Acid Vluuiira Prottixide of iron. 



Magnesia, Lime,. . . . . , ,Soda, Potash. 



These substances, we may suppose, were, in the original fluid 

 magmas from which original rocks crystallised, present in the 

 same manner in which we see them combined together in furnace 

 slags or gla.s.-^. PJach of these constituents, the alkalies excepted, 

 is of a most reiractory nature by itself, but, when several of the 

 earths unite with the silica, compounds result of various degrees of 

 fusibility, Tn this there is merely a repetition of the well-known 

 phenomena of chemical combination, where elements the most 

 antagonistic combine to form a substance innocent of any of the 

 properti-^s of its constituents. The silica or quartz, infusible and 

 chemically indiff'erent as it may appear under ordinary circum- 

 stances, acts in this case as an aei«], and, with the aid of heat, 



Vol. Y. u Xo. 3, 



