NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 261 



advisable to review in the present paper authentic data on the rela- 

 tions between the Cambrian and pre-Cambrian formations in the 

 vicinity of Helena. 



As the conclusions of Rothpletz are based primarily on the finding 

 of Cambrian fossils in a shale interbedded in what he decided to be 

 the " Helena " limestone, the first point to determine is the strati- 

 graphic position of the limestone called " Helena " m his section of 

 the formations on the eastern side of Mount Helena. If the lime- 

 stone he identified as the " Helena " is not the Helena, then the elabo- 

 rate deductions and conclusions he has drawn therefrom are without 

 foundation. 



In order that we may more clearly understand the distribution of 

 the several formations about the city of Helena, a part of the geo- 

 logical map accompanying Bulletin 527 of the U. S. Geological 

 Survey ^"is reproduced in outline as plate 39 of the present paper. It 

 should be mentioned that at the time Dr. Rothpletz made his study 

 of the rocks in that vicinity he had with him neither a topographic nor 

 geologic map, and apparently before publishing his memoir he had 

 not seen Bulletin 527 of the U. S. Geological Survey ^ containing a 

 fine geological map of the region and a description of the several 

 geological formations. The only geologic map of the area that he 

 refers to is a small scale map published by Weed ^ with all the Cam- 

 brian formations grouped under one color. He was therefore de- 

 pendent on his limited observations for the identification of the pre- 

 Cambrian, Cambrian, and later limestone formations in the Helena 

 section. 



Rothpletz speaks of the old and very poor topographic map that 

 he was obliged to refer to when writing his memoir. This is the same 

 map that was used by me when making examinations about Helena 

 in 1898, and it was on this account that the Walcott section was so 

 broadly generalized, and the exact locality of the section and of the 

 figures 3 and 4 so indefinitely stated (see p. 262). 



When I made my examinations of the formations near Helena, Mr. 

 L. S. Griswold was engaged in preparing a geologic map under the 

 direction of Mr. Weed, and in studying the Cambrian and later 

 formations more or less in detail. In view of that work, and antici- 

 pating the early publication of the results, I did not study and measure 

 the Caml:)rian and superjacent formations in detail and only collected 



^ Ore Deposits of the Helena Mining Region, Montana. By Adolph Knopf, 

 1913. 

 ^ Professional Paper 74, U. S. Geol. Survey, 1912, pi. i. 



