NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 279 



(p. II ^) that the shale on the terrace-hke shoulder is interbedded in 

 the Helena limestone, according to which the summit dolomite must 

 certainly be identical with the latter (Helena limestone). He con- 

 siders this view feasible because according to Walcott the Helena 

 limestone has a thickness of 2,400 feet (732 m.). He concludes 

 further that the Flathead quartzites mentioned by Walcott were to 

 be expected in the layer above the summit dolomite. (This would 

 be the position of the Dry Creek shale of Weed, p. 269.) 



According to Weed and the U. S. Geological Survey geologists 

 (Barrell, Griswold) who have studied the section, the summit dolo- 

 mite referred to is the Cambrian, Pilgrim limestone of Weed ; the 

 shale beneath it, the Park shale of Weed ; and the limestone beneath 

 the shale on the terrace-like shoulder referred to above, the Cambrian 

 Meagher limestone which is underlain by the Wolsey shale, and this 

 by the Flathead quartzite." 



Confusion in identification of tJie pre-Canibrian Helena limestone. 

 — Rothpletz was unfortunate in his location and identification of the 

 Helena limestone. 



My note on the section reads as follows : ^ 



Crossing the valley of the Missouri River from Whites Canyon directly 

 westward 10 miles to the Spokane Hills, on the west side of the river, one finds 

 a syncline of Cambrian resting directly on the red Spokane shales. Continu- 

 ing westward on the same line to the city of Helena, a distance of 14 miles, 

 the Cambrian sandstones are found resting on shales 250 feet above the 

 Helena limestone, or fully 3,000 feet above the contact horizon in the Spokane 

 Hills. 



Looking at the map (pi. 39), we find directly east of Mount Helena 

 the outcrop of the Helena limestone (Ah) extending across the 

 central portion of the city in a southeasterly direction to where it is 

 cut ofif by the eruptive quartz monzonite (qni). Above the Helena 

 limestone (west of it) an area of the Empire shale (Ae) occurs 

 above it and beneath the Cambrian Flathead quartzite (Cf). This 

 shale is the 250 feet of shale referred to in the above-quoted para- 

 graph. In order to avoid the quartz monzonite area and to take his 

 diagrammatic section across the strike, I carried the section south 

 of the suburb of Lenox {W-W on map) as described by him' 

 as follows : 



^Rothpletz, p. II, line 15. 

 ^ See section, p. 263. 



' Pre-Cambrian Fossiliferous Formations, Bull. Geol. Soc. of America, Vol. 

 10, 1899, p. 211. 

 *Idem. 



