434 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



clay intermixed and intercalated. It is about 2,500 feet above the base 

 of the formation. 



Resume 

 cambrian 



The greatest development of the Cambrian rocks exposed in this region 

 is in the Bighorn mountains, where they attain a thickness of 1,000 feet, 

 comprising sandstones, shales, limestones, and intraformational lime- 

 stone conglomerates. There is a relatively regular succession of beds 

 throughout, and fossils of middle Cambrian age occur at several hori- 

 zons. Probably the beds represent the Gallatin limestone and Flathead 

 formation of south-central Montana, but as they more closely resemble 

 the Cambrian succession in the northern Black hills, the name Dead- 

 wood formation is applicable to them. In the vicinity of Dead wood 

 about 500 feet of Cambrian beds are exposed, comprising buff sandstones 

 and shales in part glauconitic and the characteristic limestone conglom- 

 erates, all with middle Cambrian fossils. Formerly it was believed that 

 the upper Cambrian was also represented. The thinning of the Dead- 

 wood formation in the southern Black hills appears to be due to overlap 

 of the upper sandstone and absence of the lower beds, and farther south 

 in the Hartville uplift and for many miles along the Laramie range and 

 Rocky Mountain front the formation does not appear. 



The brown sandstones underlying the Carboniferous limestones for a 

 few miles at the north end of the Laramie range are presumably of Cam- 

 brian age, and probably mark the southern extension of the deposits so 

 prominent in the Bighorn uplift. The small outcrops of sandstone about 

 Manitou and north of Canyon City are of upper Cambrian age, and indi- 

 cate local basins, or possibly local overlaps from an irregular margin of 

 deposits which may be widespread under the plains eastward. The 

 absence of beds of middle and lower Cambrian age in Colorado indi- 

 cates that there was an extensive land area in the central Rocky moun- 

 tains during these times, a condition pointed out to this Society by S. F. 

 Emmons in 1890.* 



ORDOVICIAN 



In the Bighorn mountains, northern Black hills, and at a few detached 

 localities along the Rocky Mountain front range, the Ordovician rocks 

 appear overlying the Cambrian sandstone. In the Bighorn mountains 

 they extend continuously around the uplift, and are termed the Bighorn 

 limestone. The lower massive members, which are several hundred feet 

 thick, carry a fauna of Trenton age, and the thin, overlying, softer beds 



* Bull. Geol. Soc. Am., vol. 1, pp. 245-286. 



