220 Geological Society : Mr. R. Wallace on the 



granite; and 3rd, the upraising of the whole country, and the 

 cutting through, by water, of the granite, slate and sandstone, and 

 the formation of the Delta by the consequent debris. 



" On the Classification of Granitic Rocks." By Robert Wallace, 

 Esq. 



Assuming that granite, syenite, and other granitoid rocks, as they 

 exist in nature, agree with the definitions of those rocks respectively 

 given by mineralogists, that is, in being aggregates, though in va- 

 riable proportions, of certain determinate mineral species ; and taking 

 for granted the accuracy of the analyses which have been made, by 

 the chemists in highest repute, of the minerals which enter into the 

 composition of these aggregate rocks, the author directs his atten- 

 tion more particularly to the alkaline and alkalino-earthy ingredients 

 of those minerals ; and, inferentially, of the aggregate rocks into 

 the composition of which these minerals enter ; and finding that in 

 certain of these aggregates the alkalies exist without any admixture 

 of alkaline earth, whereas in others both alkalies and alkaline earths 

 are contained, he proposes to classify granitoid rocks according to 

 the above distinction in their chemical ingredients. 



Among the alkalies, in addition to potash and soda, he places 

 lithia ; the alkaline earths which fall under his notice, are magnesia 

 and lime. In subdividing his two principal classes of the aggregate 

 rocks, the author also takes into account the fluoric and boracic acids, 

 which appear to be essential to the constitution of certain of the 

 component minerals. 



Ternary granite, consisting either of quartz, binaxal mica and 

 felspar, or of the two former minerals and albite, is the first of the 

 aggregate rocks that comes under the author's consideration. Of 

 binaxal mica the alkaline ingredients are potash and lithia ; and one 

 of the essential ingredients of this mineral appears to be also fluoric 

 acid. Of common felspar the alkaline ingredient is potash; of 

 albite*, soda ; of glassy felspar, a mixture of potash and soda. Of 

 ternary granite, therefore, the alkaline ingredients are limited to 

 potash, soda, and lithia ; the alkaline earths, magnesia and lime, not 

 entering into its constitution. 



The different binary combinations of some two of the three mine- 

 rals, quartz, binaxal mica and felspar, belong to the second division 

 of the author's first class of aggregate rocks ; that is to say, of those 

 which contain an alkali, but not an alkaline earth. The binary com- 

 binations which he mentions, are — 



1. Common felspar and binaxal mica. 



2. Compact felspar and binaxal mica, called eurite, whitestone, 

 and felspathic granite. 



3. Common felspar and quartz, which may be either an uniform 

 mixture of the two minerals, or may consist of imperfect crystals of 

 one, or the other, or of both of them, and is then called pegmatite, 

 or graphic granite. 



4. Quartz and binaxal mica ; which, if the mica is regularly inter- 

 * Albite has been found to contain a very small variable proportion of 



lime, not exceeding 5 parts in 1000. 



