IGNEOUS 1'OltMATIONS. 



301 



exhibited by the old augite crystals, owing to resorption. Instances of 

 granular diorites in which there were two periods of consolidation are 

 known to the writer, and many oilier granular rocks may have had a similar 

 history. It is therefore doI essentially the contrasl between the products 

 of the first and second periods of consolidation which makes the porphyritic 

 structure; nor is the uniformity of mineral development characteristic of 

 the granular structure necessarily caused by consolidation within a single 

 period. 



Chemical composition. 'Pile CI II II) II isi t if 111 of tile \allllollt doh'l'ite is giVCH 



under I, and that tit' the augite from it under II. Both analyses are by 

 L. Gr. Eakins. 



Analyses ofdolerite and augite from Valmont, Colo. 



The rock analysis shows a normal composition excepting for the 

 alkalies, but the predominance fit' potash over soda confirms the determi- 

 nation el' orthoclase and explain- its presence. This relationship of the 

 alkalies is also found in the other basalts to be described, and is the leading 



characteristic of these rocks. The Occurrence of orthoclase in liasalt lias 



occasionally been announced, bul not often with chemical evidence sustain- 

 ing the determination. In the present case it is entirely natural that the 



