THE SEDI^rEXTARY PLATFOIUf. 395 



ally, however, a stratum containing limestone nodules full of fresh-water 

 shell remains. The si)ecies are not of specitic value, consisting of Vivi- 

 pariis and Goniobasis, with Unios, but are of interest as indicating the 

 fresh-water nature of the deposit. 



The coal series of sandstones, clays and marly l)eds is overlaid by the 

 dark carbonaceous shales referable to the Fort Benton, and containing 

 fossils of that age at the base of the mountains. No Dakota is recogniz- 

 able, either lithologically as the conglomerate, which forms so prominent 

 a feature of the strata between Jurassic and Fort l^enton sediments farther 

 south, or paleontologically by fossils. If present, it is so like the under- 

 lying formation as to be indistinguishable from it. 



In Belt butte, a prominent eminence rising above the benchland west 

 of the mountains, the shale series is well exposed and appears of Fort 

 Benton facies, although the sandstones indicate shallow-water conditions 

 not shown by tlie Missouri River l)eds. The lower beds are black and 

 deep purple shales, with occasional thin layers of sandstone. There is a 

 prominent sandstone horizon forming the girdle or belt around the butte 

 which is also a persistent horizon about the mountain flanks. Above it 

 the shales are grayer in color, and the arenaceous varieties which succeed 

 them carry large quantities of Ostreas. 



The highest beds of the unaltered sedimentary series are white or light 

 colored sandstones alternating with argillaceous strata, which on the north 

 flanks of the mountains carry fresh-water fossils. This horizon is seen 

 forming prominent bluffs along the east end of the Shonkin Sag and the 

 higher bench of Square butte, and is probaljly the same l^ed seen capped 

 by gray shales at Highwood ga}). This sandstone is tlie liighest horizon 

 of unaltered rocks seen in the Highwoods. It is suggested that it may 

 represent the extreme soutliward development of the non-marine Ik'lly 

 River formation, an infra-Fort Pierre grouj) of coal-bearing rocks of tlie 

 Canadian geologists. 



About each of the volcanic cores of tlie Highwoods there are areas of 

 highly altered metamorphosed sediments, which in South peak form the 

 main mass of the mountain. These rocks have lost all trace of original 

 bedding planes, and consist of dense hornstones, quartzites, etcetera, and 

 only show in their i)resent character that tliey were originally argillaceous 

 and arenaceous shales. They cannot be distinguished in their present 

 state from the baked Algonkian slates or altered Cambrian shales of 

 Ca.stle mountain or the Livingston beds of the Crazy inouiitnins, features 

 of local metamorphism in areas to the southward. 



IGNEOUS ROCKS. 



General Characters. — The individual characteristics of the v^olcanic con- 

 duits are such as to warrant l)rief notes U[)()n each (•rui)tiv(' center. A 



