THE ORIGIN OF IGNEOUS ROCKS. 
131 
mountains is approached the augites assume a stronger 
green, and many varieties of rock contain grass-green augites 
and many others true segerine and acmite, with their char¬ 
acteristic green and brown colors and peculiar crystal forms. 
In other districts, better known in Europe, groups of rocks 
are distinguished by purplish augites. 
The relation which was pointed out by Judd * in 1876 as 
existing between the rocks of Hungary and those of Bohemia 
also exists between the volcanic lavas of the Great Basin of 
western America and the belt of country east of the Rocky 
mountains, including the vicinity of the Highwood moun¬ 
tains and the Crazy, mountains in Montana; the Black hills 
in South Dakota, and Leucite hills in Wyoming and certain 
parts of Colorado. 
In the region just mentioned are found phonolites, leucite 
rocks, and trachyte, with allied rocks, such as occur in Bo¬ 
hemia, Germany, France, Italy, the northwest portion of 
Africa, and the islands off its western coast. The. rocks of the 
alkali belt in America have not been thoroughly exploited, 
so that our knowledge of them is very imperfect. We are 
better acquainted with those of the Great Basin and Pacific 
# coast volcanoes, which are identical with the rocks of Hun¬ 
gary. 
The variation of the pyroxene has been shown by Merian f 
to depend on the chemical constitution of the rock. The 
increase in the green color indicates the greater proportion 
of the acmite molecule, which carries the soda. The purplish 
augites are found in the rocks very rich in ferromagnesian 
oxides and titanic acid and characterize these groups, being 
most noticeable in some nephelinites. It is not to be under¬ 
stood that every rock in the region in which these varieties 
of augite occur is characterized by a particular kind of augite. 
On the contrary, there are districts where all of them appear 
* Judd (J. W.) On the Ancient Volcano of the District of Schemnitz, 
Hungary. Quart. Jour. Geol. Soc. 8° London, 1876, vol. 32, p. 303. 
t Merian (A.) Studienan gesteinsbildenden Pyroxenen. Neues Jahrly 
Min., etc, '8°, Stuttgart, 1884, Sup. vol. 3, pp. 252-315. 
