106 /. P. IDDINGS 



some parent magma by some process of chemico-physical separa- 

 tion or differentiation. 



Besides genetic series that are chemically quite distinct from 

 one another there are others much less so, and others that resemble 

 one another closely. Petrographical provinces are in some cases 

 strongly contrasted chemically. But in certain regions there 

 appears to be a gradual shifting in chemical composition of the 

 rocks from one locality to another. This is well illustrated in 

 the region embracing the Yellowstone National Park, the Crazy 

 Mountains, Castle Mountain, Little Belt Mountains, Highwood, 

 and Bearpaw Mountains in Montana. 1 The series of rocks at 

 each of the localities named becomes relatively richer in alkalis 

 from the south northward, potash assuming a very prominent 

 role. The analyses from these localities to the number of 175 

 are compared with one another in Diagram 5, which shows the 

 gradual shifting of the alkali-silica ratios. 



In general a more or less gradual shifting of chemical char- 

 acters with increase in alkalis obtains for genetic series of rocks 

 from the Great Basin in Nevada, Idaho, and Utah eastward 

 across the Rocky Mountains to the Black Hills, S. D., Leucite 

 Hills, Wyo., Cripple Creek, Colo., into Arkansas and southward 

 into the Transpecos region of Texas. Throughout this vast 

 region there are innumerable genetic series of igneous rocks. 

 The series from somewhat remote parts of the region are chem- 

 ically quite clearly distinguished, but series from intermediate 

 localities grade into one another so that there are in reality ser- 

 ies of series. The term series in the sense here used applies to 

 all igneous rocks of one petrographical province that belong to 

 one period of volcanic activity, which may be of very great 



1 Iddings, J. P., The Eruptive Rocks of Electric Park and Sepulchre Mountain, 

 Yellowstone National Park, I2th Ann. Rep. U. S. Geol. Survey, 1892, pp. 569-664. 

 Origin of Igneous Rock, loc. cit. Absarokite-Shoshonite-Banakite-Series, Jour. 

 Geol., Vol. Ill, Chicago, 1895, P- 935- 



Weed, W. H. and Pirsson, L. V., Geology of the Castle Mountain Mining 

 District, Montana, loc. cit. Highwood Mountains of Montana, Bull. Geol. Soc. Am., 

 Vol. VI, Rochester, 1895, PP- 389-422. The Bearpaw Mountains of Montana, Am. 

 Jour. Sci., Vol. I, 1896, pp. 283, 351 ; Vol. II, pp. 136-188. Also Bull. U. S. Geol. 

 Survey No. 148, Washington, 1897, pp. 1 17—136, 142-157. 



