BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 3, pp. 445-452 August 8, 1892 



THE QEOLOGY OF THE CRAZY MOUNTAINS, MONTANA. 



BY J. E. WOLFF. 



(Read before the Society December 29, 1891.) 



CONTENTS. 



Page. 



Topography 445 



< ieneral Structure , -446 



Eruptive Rocks of the northern Area 449 



Structural Aspects 440 



Lithologic ( Iharacters 450 



Features of tin.* Southern Area 4">o 



Topography. 



The Crazy mountains arc situated in centra] Montana, centering about 

 latitude 46° 3 longitude 11"° 15'. Theyforra a high isolated range of the 

 Rocky mountains, lying about 30 miles cast of the easterly border of 

 the main mass of the mountains, and rise abruptly from the eastern table- 

 lands, attaining an extreme elevation of about 11,000 feet above sea-level. 

 The Yellowstone river flows around their southern end a few miles after 

 its exit from the mountains at the lower canyon, and the range is there- 

 fore in plain view from the Northern Pacific railroad for many miles 

 eastward from the town of Livingston. 



The mountains trend a little west of north and arc about 40 miles long 

 and 15 or 20 wide. A Large branch of the Yellowstone, called Shields 

 river, which Hows southward along the western base, has cut a deep. Hat 

 transverse valley at its head nearly through to the eastward drainage; 

 and there divides the range into northern and southern halves. Of the 

 two i >oi-t ion- thus defined the southern reaches the greater elevation. It 

 has numerous sharp peaks, often of a jagged aiguill type, and the arrange- 

 ment of the drainage is distinctly radial, since the streams How westward, 

 southward and eastward from the central mass of high peaks. In moving 

 lip one of these streams toward t lie head wr lind the valley at first com- 

 paratively broad, bounded by high bluffs of nearly horizontal sandstones, 



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