4:4:2 Pirsson and Washington — Geology of New Hampshire. 



sideration this would become biotitic grano-liparose. In the 

 prevailing terminology it would be a granite aplite. 



Alkalio character of the rock. — The alkalic nature of rocks, 

 which, like this one, are of high silica content, is to a great 

 extent masked by the large dilution with silica. In a general 

 way we might compare magmas to mixed solutions of salts in 

 water. The ratios of the salts or the oxides which compose 

 them and which give the mixture its general chemical char- 

 acter remain the same whether the compound solution be 

 dilute or concentrated. While the analogy with molten mag- 

 mas is not exact, since the silica plays a role different from 

 water, it will serve in a general way, as the ratios of the 

 metallic oxides are a very characteristic feature. In rocks 

 containing about 55 per cent of silica, the expression of the 

 alkalic nature becomes most evident, so that in speaking of 

 alkalic rocks petrographers are apt to have in mind those 

 which contain about this percentage of silica. Yet rocks with 

 high silica percentages may be as alkalic relatively to the other 

 components as with low. This may be illustrated as follows. 

 Let us take the liparose just described and reduce the silica to 



I. II. 



Si0 2 75-65 55-02 



A1 2 U 3 12-89 23-14 



Fe 2 3 0-89 1-60 



FeO 1-11 1-99 



MgO 0*20 0-30 



CaO 0*48 0-86 



Na 2 3-71 6-66 



K 2 U 5-50 9-87 



H 2 0-23 0-41 



TiO„ 0-05 0-09 





la. 



Ha. 



Quartz . 



31-02 







Orthoclase 



32-80 



58-38 



Albite.- 



31-44 



4-19 



Nephelite 



. . _ _ 



28-12 



Anorthite 



1-95 



3-89 



Diopside 



0-46 





Hypersthene - 



1-46 







Olivine 





2-11 



Magnetite 



1-39 



2-32 



Water, etc 



•28 



•62 



Total..... 100-71 100-00 Total 100-80 99*63 



55 per cent, or in other words, we will subtract 45 per cent of 

 silica from the magma, leave all the other oxides in the original 

 proportions, and reduce the whole to 100. The result is seen 

 in columns I and II of the above table, and their calculated 

 norms, or those minerals which they would naturally form if 

 in a state of dry fusion without the aid of the mineralizers neces- 

 sary to condition the formation of such alferric minerals as 

 mica and hornblende, are seen in la and Ha. II and Ila 

 show the chemical and mineral composition of a characteristic 

 " f oyaite," of a somewhat potassic character. To change this 

 to the granite of I it is necessary to dilute it with 82 per cent 

 of its own weight of silica, a figure which shows the great 

 amount of dilution the alkalic magmas of high silica content 



