THE ORIGIN OF IGNEOUS ROCKS. 139 
In the group at Crandall basin the variation of the alkalies 
is still quite gradual, but the ratio of the soda and potash 
molecules varies from about 2 :1 to nearly 1:1. There is a 
greater proportion of potash, and the total percentage of 
alkalies is higher. In the special group of dike rocks from 
the Absaroka range and the exceptional flows associated with 
them the molecules of potash are more nearly equal to those 
of soda, and the total amount of alkalies is still higher than 
in the previous groups. It is known that in the regions 
north and east of this the amount of alkalies increases very 
considerably in certain groups of rocks, but no systematic 
chemical study has been made of them. 
Comparing the groups of analyses just given with those 
of Vesuvius and its vicinity, a marked difference is at once 
observed. In the latter group the variation of the molecules 
of the alkalies is more irregular, though they maintain a 
generally uniform proportion, which is indicated by their 
nearly horizontal position in the diagram; but the potash is 
more uniform than the soda, which increases from the less 
silicious to the more silicious end of the series. The ratio 
between the potash and soda varies from nearly 4 :1 to 4 : 7. 
In general the potash is much greater than the soda, which 
is the distinguishing characteristic of this group. In the 
four groups already mentioned the alumina, though quite 
variable, maintains a high average except in certain parts of 
the rocks in the Absaroka range and Crandall group. 
In the rocks of the Etna group the variation of the alka¬ 
lies is quite irregular, and the soda is in excess of the potash, 
the ratio of the soda to potash ranging from 8 :1 to about 
1:1, while in the more silicious rocks from Pantelleria the 
alkalies are more uniform, and the soda is greatly in excess 
of the potash, varying from about 2 : 1 to 5:1, the total 
amount of alkalies in the Vesuvian and Pantellerian groups 
being much greater than in the rocks of the Yellowstone 
Park region. In the rocks of Etna and Pantelleria the alu¬ 
mina varies greatly for a slight range of silica, in both groups 
falling rapidly toward the more silicious end of the series. 
