NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 265 



suburb of Lenox for 2.5 miles (4 km.) to where both formations are 

 cut off by a mass of rhyohte breccia about a mile (1.6 km.) north of 

 the old mining" railroad station of Montana City. 



We will now consider the Algonkian and Cambrian formations as 

 they were known in the vicinity of Helena, by publication in 191 3, also 

 a section of the Cambrian formations 20 miles {t,2 km.) northeast 

 of Helena and in the Little Belt and Big- Snowy Mountains, in order 

 that we may better understand the Mount Helena section and the 

 Rothpletz interpretation of it. 



ALGONKIAN AND CAMBRIAN SECTIONS AT HELENA 

 AND VICINITY 



In Bulletin 527 of the U. S. Geological Survey,' Dr. Adolph Knopf 

 publishes a description of the sedimentary series at Helena, the data 

 for which are taken largely from- an unpublished report by 'Sir. 

 Walter Harvey W^eed.' 



PRE-CAMBRIAN ALGONKIAN FORMATIONS 

 In speaking of the ])re-Cambrian Algonkian formations Knopf 

 says : " 



In the Helena district only the four uppermost formations are found, and the 

 top of the Marsh shales has been eroded before the deposition of the Cam- 

 brian, so that the upper limit of the formation and its entire thickness are not 

 known. 



The four formations mentioned are as follows, from below upward : 



Spokane shale. — (As on map.) It consists of massive- and thinly- 

 bedded, siliceous shales, usually of a deep-red color, but passing in 

 places into green and gray rocks, containing arenaceous beds merging 

 into sandstone. The rocks form low hills, bordering the Prickly 

 Pear Valley to the northwest of Helena, and a small outcrop occurs 

 between East Helena and Helena between the railway tracks' and 

 Prickly Pear Creek. 



Empire shale. — (Ae on map.) The Empire shale is a shale com- 

 posed of massive-bedded, greenish-gray, well-banded, siliceous shales, 

 showing color bandings of light and dark material and locally a 

 marked knotty structure. 



These shales underlie a considerable area of North Helena and 

 the subdivision known as Kenwood, and extend around to the north 



^ Ore Deposits of the Helena Mining Region, Montana. Knopf, 1913, pp. 

 86-94- 



" Idem, p. 86. ^ Idem, p. 87. 



