I9I1.] IDDINGS— PROBLEMS IN PETROLOGY. 291 



while the third is ahiiost wholly hornblende, only 2 per cent, being 

 pyroxene and nephelite. The same magma might have crystallized 

 as nephelite-basanite, as appears from the calculated mineral com- 

 position shown in column 5, and from comparison with the analysis 

 and mineral composition of a nephelite-basanite from Colfax County, 

 N. M., shown in columns 4 and 6. 



This is only an extreme case of variations well known to exist in 

 most groups of rocks that ma}- be referred to chemically similar 

 magmas. And the magma alrcad}- cited as capable of furnishing a 

 pyroxene-andesite may also yield a quartz-mica-diorite, whose com- 

 position is shown in column 3 of the first table. 



It is evident from these examples that the minerals called horn- 

 blende, or more properly amphibole, in the descriptions of these 

 rocks differ widely in chemical composition, and often represent 

 totally different mixed salts. Thus in the hornblendite of Gran, the 

 hornblende contains all the components that might, under other con- 

 ditions, have crystallized as pyroxene, olivine, feldspar, leucite, 

 nephelite and magnetite. 



Any attempt to correlate igneous rocks on the basis of the actual 

 mineral composition, without taking into account the actual chem- 

 ical composition of the minerals involved in each case must lead to 

 confusion. 



One of the most important problems in petrology is the elucida- 

 tion of the laws controlling the production of mineral compounds 

 from molten magmas. A consideration of the simpler chemical reac- 

 tions that may be expected to take place in silicate solutions like 

 rock magmas, and which do take place in crucibles in the laboratory, 

 explains the formation of the feldspars, leucite, nephelite, quartz, 

 diopside, hypersthene, olivine, magnetite and some other rock 

 minerals. 



Minerals like mica clearly involve the chemical action of water, 

 or its components, hydrogen and hydroxyl. since hydrogen enters 

 into its constitution. According to Penfield hydroxyl, and sometimes 

 fluorine, enters into the composition of hornblendes, forming bivalent 

 radicles with aluminium, and ferric iron. In pyrogenetic analcite, 

 and in other possibly primary zeolites in igneous rocks, H^O enters 



PROC. AMER. PHIL. SOC. L. I99 S, PRINTED JUNE 30, I9II. 



