Washington — Statement of Rock Analyses. 61 



A rock analysis, it must be remembered, is primarily 

 intended for, and almost exclusively used by, petrographers. 

 Therefore for him an arrangement on analytical or strictly 

 chemical lines is neither advantageous nor appropriate. What 

 he needs especially is an arrangement which shall bring the 

 essential chemical features — both the percentage figures and 

 the molecular ratios — prominently and compactly before the 

 eye, so that the general chemical character and the relations of 

 the various constituents may be seen at a glance. It is also of 

 importance that the arrangement be such as to facilitate com- 

 parison of one analysis with another. 



To the petrographer the eight oxides, Si0 2 , A1 2 3 , Fe 2 3 , 

 FeO, MgO, CaO, Na 2 0, and K 2 0, which in practically every 

 case are present in preponderating amount, are, and must 

 always remain, of prime importance. They are the oxides 

 which, by their relative amounts, determine the chemical char- 

 acter of the rock. H 2 and C0 2 , which are also found some- 

 times in notable quantity, are chiefly of value as a measure of 

 the freshness of the rock. The other constituents, while of 

 great interest, are present in minute quantities and in general, 

 especially as compared with the main oxides, influence the 

 character of the rock only to a very limited extent, either 

 entering into the composition of accessory and subordinate 

 minerals, or replacing to a very small extent the more 

 important oxides in the essential minerals. 



Hillebrand's plea* for their determination is well founded, 

 and it is of course a desideratum that all analyses should be 

 complete as to the rarer constituents. This, however, is not 

 always possible, and it will probably remain true as it is at 

 present, that many otherwise good and serviceable analyses are 

 incomplete in this respect, or only show determinations of a 

 few of the rarer constituents, notably Ti0 2 , MnO, P 2 5 , 

 and CI. 



By putting the eight main oxides together, then, the petro- 

 grapher is able to see at a glance the character of the rock 

 under study, and the molecular ratios of these oxides, which 

 are the only ones of practical importance, may be written after 

 them and so easily compared inter se. Furthermore, whether 

 an analysis is complete or incomplete, these oxides, which are 

 determined in every case, are always in the same relative posi- 

 tion, so that the eye finds them without trouble, thus immensely 

 facilitating comparison and study. 



The question naturally arises as to the advisability of stating 

 the analytical results, not in oxides, but in terms of the ele- 

 ments, oxygen being given as a separate constituent. This is 



* Hillebrand, Bull. 148, U. S. G. S., p. 15, 1897. 



