288 WINTHRQP P. IIAYNES 



longer applicable, since they have been shown to be due to sub- 

 aerial and fluviatile deposition rather than to lakes. 1 



The Bozeman formation here is chiefly of Miocene age, but in 

 some parts of the region strata of Oligocene (White River) age have 

 been identified. 



TLEISTOCENE 



The hills in this region were evidently too low for local glaciation 

 and no signs of regional glaciation have been observed as far south 

 as this in Montana. Gravel terraces, along the rivers indicate 

 greatly increased stream action in Pleistocene times. 



IGNEOUS ROCKS 



The igneous rocks in the region north of Three Forks are rela- 

 tively unimportant, and are in the form of rather small intrusions 

 of three different rock types. 



GRANITE 



About two miles west of Lombard, in the double horseshoe bend, 

 the Missouri River flows for a short distance through a gorge cut in 

 an intrusive mass of granite. Only the eastern boundary of this 

 granite could be accurately mapped, but the approximate western 

 limits are noted on the map. 



The granite is of a light-gray color, with a medium line texture 

 and a somewhat porphyritic structure. The minerals recognized 

 in a megascopic examination are white and grayish feldspar some- 

 what kaolinized, quartz in small amounts, and hornblendes mostly 

 altered to chlorite. Under the microscope the feldspars are seen 

 to be deeply kaolinized, but are chiefly orthoclase with some albite. 

 There is a considerable amount of hornblende which is altered in 

 part to chlorite and epidote. Some biotite and magnetite are also 

 present. 



In places this rock is almost entirely without quartz and there- 

 fore grades into a syenite. It seems to correspond closely with the 

 description of the syenite of Yogo Peak 2 and vicinity in the Little 

 Belt Mountains, which is noted as grading into a granite-syenite- 



1 H. F. Osborne, Bull. U.S. Geol. Survey. No. 361, 1909, p. 28. 



2 Atlas Folio, U.S. Geol. Survey No. 56, 1899. 



