308 THE CANADIAN NATURALIST. [Sept. 



o 



aive them the comnositioa of bi-iiilicates, in which two atoms of 

 silica arc present for every one of mon-oxide, and six for every 

 two of sesqui-oxide, or in which the ox3'gen ratio between bases 

 and acid is as one to four. The orauular, porphyritic and 

 trachytic developenients of those rocks are respectively represented 

 by granite, felsitic porphyry and rhyoiite. Proceeding in the same 

 manner as with the neutral rocks we find the following among this 

 series to approach most closely in composition to bi-siiicates : 



O. IJATI.'). Quantity of 



Miica In I'hi 

 B.iScs, Si]ic';t. parts rock. 



I. Granite from Heidelberg,— Streii, a I :J 893 72.11 



n. Granite from Doochary Bridge, 



Donegal,— Houghton 1 3.7GU 72.21 



III. Granite of Fox Eock, near 



Dublin, — Uougbtou 1 4.077 7Ll 



IV. Granite of Striegan near Silesia, 



—Strang 1 4.3G4 73.13 



Y. Granite of Blackf>tairs Moun- 



tian, Wexford,— Houghton .. - 1 3.953 73.20 



Average I 4 009 72.73 



I. Felsitic porphyry from Miihlberg 



near Halle, — Laspeyres 1 4.051 72.24 



I. Quartzose traehyt from Hohen- 



burg, near Berkum, opposite 



the Siebeugebirge, Bischof .. 1 3.824 72.23 



II. Quartzose trachyte from the I.<- 



landofPonza. — Abich 1 4.152 73.40 



Average 1 3.988 72.86 



It appears, therefore, that the oxygen ratio 1 to 4 corresponds 

 to an average silica percentage of 72.61, and to such bi-silicato 

 rocks the name silicic might be applied. 



But besides this silicic series of rocks there is found another 

 series of very different chemical con.stitution, and in which the 

 bases, and not the silica, preponderate. It is only, however, in 

 rare instances among these rocks that the silica disappears to such 

 an extent as to form a disilicate, i.e., a compound of one equivalent 

 of silica with two of base, or in which the quantities of oxygen 

 contained in acid and base are equal. A very well marked series 

 of basic rocks may, however, be pointed out in which two equiva- 

 lents of silica are combined with three of base, and in which the 

 oxygen ratio is as IJ- to 1. The rocks which represent this 

 basic development of the porphyritic and trachytic textures, 

 are, respectively, augitic porphyry and nephelinite. The 

 following are instance of these rocks in which the oxygen ratio 

 most closely approaches 1*333=1 : — 



