Invasion to Regional Metainorphism. 7 



♦ ages, but are younger than early or middle Tertiary. 

 Tliey seem, therefore, closely related in age to the Lara- 

 mide revolution, their intrusion beginning in the Creta- 

 ceous and probably extending into the Eocene. The Idaho 

 batholith is 250 miles long by 80 miles wide. The Boulder 

 batholith outcrops 80 miles east, showing a length of 60 

 miles, and a width of 18. Between the two are various 

 smaller batholiths and stocks of irregular form and dis- 

 tribution. Still farther east 80 to 100 miles, in the Little 

 Belt quadrangle, occur the Castle Mountain and Crazy 

 Mountain stocks. Thus there is an irregular dying out 

 of surface exposures from Idaho toward central Montana. 

 But the Little Belt region is particularly instructive in 

 giving suggestions of a broad underground occurrence of 

 intrusions. Numerous dikes occur, but the most striking 

 feature is found in the south central portion of the quad- 

 rangle. Here a mountain rises to an elevation of 8200 

 feet, showing no igneous core, but rivalling in elevation 

 the mountains made by the Crazy Mountain stocks and 

 their aureole of metamorphosed sediments. This moun- 

 tain, like those possessing central stocks, has on its flanks 

 a zone of radiating dikes. The physiographic form 

 points to a local hardening of the sedimentary formation, 

 a zone of metamorphism which, as shown around the other 

 stocks in the Livingston formation, is equally or even 

 more resistant than the igneous rock in the center. The 

 dikes, to those who seek their meaning, look inward to a 

 hidden igneous body, like the statues of the Alhambra 

 legend ^\^ose eyes f ocussed on the spot of buried treasure. 

 Interpreting the Crazy and Castle Mountain stocks in 

 accordance with these indications of hidden magma, as 

 well as by their own broad metamorphic girdles, leads 

 to the conclusion that they must widen downward, and 

 that much of the region is underlain by a broad batholith 

 of which only the roof-dikes and the greater cupolas show 

 upon the surface. The irregular flexures of the Liv- 

 ingston formation are due to the irregular sagging and 

 doming of the cover of a magma tic chamber now solidi- 

 fied. Some compression is shown, but the chief feature 

 of the deformation is its irregular vertical w^arping. 



To the south, in the Yellowstone National Park, Hague^ 

 has shown that many of the stocks are not volcanic necks 

 and never reached the surface, and Daly' has presented 



" Arnold Hague, Science, new ser., 9, 425-442, 1899. 



^E. A. Daly, Proc. Amer. Acad. Arts and Sci., 47, 63, 1911. 



