Reviews 



Geology of the Little Belt Mountains, Montana, with ?iotes on the 

 Mineral Deposits of the Neihart, Barker, . Yogo, and other Dis- 

 tricts, by Walter Harvey Weed, accompanied by a report 

 on The Petrography of the Igneous Rocks of the District, by L. 

 V. Pirsson. Twentieth Annual Report of the U. S. Geo- 

 logical Survey, Washington, 1900. Pp. 257-581, with 42 

 plates and 43 figures. 



This voluminous report is based upon field work done in Septem- 

 ber 1893, and August 1894. The major part of the report was writ- 

 ten by Mr. Weed, who describes the position, topography, and geo- 

 logical structure of the region. The rock formations are described in 

 succession from the gneisses, schists, and Algonkian series, through 

 Cambrian and Siluro-Devonian to Carboniferous. The geology of 

 the region is treated in detail, commencing with the southern part of 

 the range, and followed by Judith area, the Yogo, Big Baldy Moun- 

 tain, Wolf Butte, Taylor Peak, Barker, and Monarch districts, and 

 finally the Neihart. 



A chapter is devoted to the general geology, and deals first with 

 the history of the region as interpreted from sedimentary rocks, show- 

 ing that the same general conditions prevailed in this region as in the 

 rest of the eastern part of the Rocky Mountain area of the state. The 

 uplift of the Little Belt Mountain range is supposed to have been due 

 to lateral compression, but the minor doming and faulting observed at 

 all the larger mountain masses are due to igneous intrusions. 



With the exception of Yogo Peak all the prominent mountain 

 masses are formed of igneous rock, whose structural features show that 

 they constitute a group of closely related forms, grading from typical 

 laccoliths to those of plutonic plugs or bysmaliths. The largest of 

 these bodies is exposed over an area of three by five miles, and is 

 probably 3000 feet thick. With these are associated numerous intru- 

 sive sheets, but few dikes. There are besides several intrusive rocks 

 not directly connected with the laccolithic bodies. The character and 

 origin of these intrusions are discussed. 



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