NO. 4 CAMnRIAX AND PRE-CAMP.RI AN AT HELENA 281 



tinuation of the pre-Cambrian Helena limestone. This contact ex- 

 tends on a north-northeast and south-southwest line for over i,ooo 

 feet (305 m.). 



To one who had seen the Cambrian limestones on Mount Helena 

 and mistaken them for the pre-Cambrian Helena limestone, it would 

 be most natural to conclude that not only were the Cambrian lime- 

 stones the same as the pre-Cambrian Helena limestone, but that the 

 great mass of the Helena limestone southeast of the fault B-B w\is 

 also of the same geological age as the Cambrian limestones. 



Rothplcts interpretation of Weed's Cambrian as given in Butte 

 report. — In speaking of the Cambrian section described in Weed's 

 report upon the Butte district/ Rothpletz states in substance that, 

 according to Weed, immediately beneath the black Jefiferson lime- 

 stones the shales and limestones of the Cambrian with their basal 

 quartzites rest on the shales and interbedded sandstones of the Belt 

 formation, and further,' that " Walcott has, however, separated a part 

 of this thus (above) delimited Cambrian, namely, the Helena Hme- 

 stone and the Marsh shales, and assigned it to the Belt formation. 

 What then still remains over of Weed's Cambrian at Helena is not 

 stated except that the Flathead sandstone [quartzite] forms the base 

 of the Cambrian." 



I do not understand this reference of Rothpletz to the Helena 

 limestone and Marsh shale, as Weed has never to my knowledge in- 

 cluded the Helena limestone and the Marsh shale in the Cambrian. 

 It serves to further illustrate the confusion in the mind of Rothpletz 

 as to what the Cambrian limestones were, also as to the Helena lime- 

 stone. In all the publications of Weed and Walcott, the Cambrian 

 limestones are considered to be above the Flathead quartzite and the 

 Helena limestone unconformably beneath it. 



Comparison of Rothplet:: and Weed Cambrian sections in the 

 Helena district. — Placing in comparison the Rothpletz and Weed 

 sections in the Helena district we have the following result : 

 Rothpletz Weed 



6. Limestone (Helena). Pilgrim limestone 317 ft.-j 



5. Shales (Capitol Creek). Park shale ...150" j 



4. Limestone (Helena). Meagher limestone 400" j-Cambrian. 



3. Shales (Empire). Wolsey shale 420" 



2. Sandstones (Quartzite). Flathead quartzite 300 " j 



I. Shales. Marsh shale \ . 



Helena limestone j 



' Geology and Ore Deposits of Butte District. Montana. Prof. Paper 74. 

 U. S. Geol. Surv., 1912. 

 ^ Rothpletz, p. 7, line 9. 



