800 H. S. WASHINGTON DECCAN SALTS AND PLATEAU BASALTS 



plateau basalts the combined iron oxides would amount to about 11 per 

 cent or more, and this is the more marked if only the most abundant 

 group of the more femic basalts are considered. I am inclined to think 

 that the comparatively high ferric oxide shown in Daly's average is due 

 in part to oxidation of ferrous oxide through slight alteration and in 

 part to defective determination of the ferrous oxide — a not unusual 

 analytical error. The percentage of titanium dioxide is appreciably high 

 in the plateau basalts. 



It would thus appear that the plateau basalts differ from what might 

 be called the cone basalts essentially in the higher iron and titanium 

 content of the former and possibly in the relatively less oxidized condi- 

 tion of the iron. This must be considered as a broad general distinction. 

 Examples may be found among typical plateau basalts in which the iron 

 oxides are not specially high, just as examples may be found among cone 

 basalts in which the iron oxides are much higher than the average. Min- 

 eralogically, as we have seen, this chemical difference is expressed in the 

 presence of highly ferromagnesian hedenbergitic enstatite-augite in the 

 plateau basalts, in contrast to that of highly calcic or diopsidic augite in 

 the cone basalts. It may also find expression in the striking tendency of 

 the augite and magnetite to be among the last minerals to crystallize ; so 

 that the glass present in the not wholly crystallized plateau basalts would 

 have a composition corresponding to a mixture of augite and magnetite, 

 examples of which we have seen on the island of Skye, in Colorado, and 

 possibly elsewhere. 



CLASSIFICATION OF BASALTS 



It would follow from the considerations just mentioned that basalts, in 

 general, may be referred to at least two different groups : the plateau 

 basalts, which are high in iron, and the cone basalts, which are low in 

 iron and relatively high in magnesia and lime. This does not preclude, 

 it must be said, the possibility of the recognition of other groups of 

 basalts, such as, let us say, those high in soda and grading into nephelite 

 tephrites, or those high in potash and grading into leucite tephrites. 



No hard and fast line can be drawn. As Iddings showed many years 

 ago and as continued and continuous study of igneous rocks has abun- 

 dantly proved to be the case, igneous rocks grade into each other in all 

 possible ways. We are not dealing in the classification of rocks, with 

 more or less definitely classifiable separate individual entities, as is true 

 of plants and animals, in which the individual records the progress and 

 the direction of evolution, and the characters of which are brought about 

 very largely through the necessity for the means of adaptation of exist- 



