CORRELATION OF SECTIONS 17 



deposited, in part at least, in the erosion interval between the Algonkian 

 and the Middle Cambrian. 



Correlation of Sections 



The most easterly section, that of the Belt mountains, has more lime- 

 stone in proportion to the arenaceous matter and, with the exception of 

 the Neihart sandstone at the base, finer sediments; these conditions 

 indicate that the sediments were derived mainly from a somewhat distant 

 source of supply. One horizon of this section, the Newland limestone, 

 is marked by the presence of fossil crustaceans that also occur in the sec- 

 tions 200 miles to the northwest, as discovered in the Lewis range by 

 Messrs. Willis and Weller,* and Doctor E. A. Daly on the forty-ninth 

 parallel, f 



Tracing the upper formations of the Belt terrane north of Helena, we 

 find, at Lewis and Clark pass on the Continental divide, a series of red- 

 dish, arenaceous rocks beneath the Cambrian, with some limestone. This 

 is 40 miles from the typical section east of Helena, and there is a manifest 

 change in the sediments, especially in the presence of a greater thickness 

 of arenaceous beds between the base of the Cambrian and the Helena 

 limestone. At Helena the Marsh shales are 300 feet thick, and similar 

 beds at the Lewis and Clark pass give a thickness of 1,015 feet. 



The Helena Limestone series in the vicinity of Helena has an estimated 

 thickness of 2,400 feet. It has numerous arenaceous and siliceous bands 

 interbedded with the limestone. By the elimination of a relatively small 

 amount of the calcareous matter the greater part of the limestones would 

 disappear, and a section much like that of Lewis and Clark pass replace 

 the Helena. Limestone series. 



Ten miles north of the Lewis and Clark Pass section the limestones 

 of the Dearborn section corresponding to those of the Helena series are 

 well developed. The reddish colored shales of the Marsh formation 

 appear to be absent, the Cambrian resting on buff and gray arenaceous 

 beds below the reddish beds of the Lewis and Clark Pass section. The 

 Helena limestone series is represented by 435 feet of siliceous limestones. 

 Below the latter a great thickness of greenish and purplish tinted arena- 

 ceous and siliceous beds extend downward 5,700 feet before any more 



♦Bull. Geol. Soc. Am., vol. 13, p, 317. 



t Doctor Daly writes me that the locality is at a point on Oil creek about 6 miles 

 east of the Boundary monument at the summit of the Rocky mountains and about 4 

 miles north of the line. Through the courtesy of the Geological Survey of Canada, by 

 Doctor Daly, I had the opportunity of examining the specimens. They are identical 

 in appearance and form with those from the Newland and Altyn formations. 



Ill— Bull. Geol. Soc. Am., Vol. 17. 1905 



