NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 267 



gulch east of Lenox, the 4,350- foot knoll [on topographic map] at 

 that locality showing light-gray, fine oolitic limestone with black 

 grains in a white matrix." ^ 



Marsh shale. — {Am on map.) The Alarsh shale consists of red 

 and yellowish-green shales and thin-bedded sandstones. 



CAMBRIAN FORMATIONS 



These formations were studied by Weed on Mount Helena and 

 they compare quite closely with the same formations as published by 

 Weed in 1900 in his account of the Geology of the Little Belt Moun- 

 tains of Montana^ (see p. 273). 



From the base upward the following is the order of succession at 

 Mount Helena ' : 



Flathead quartsite.^ — {-€f on map.) " The Flathead quartzite, 

 the lowest formation of the Cambrian system, consists of a hard, fine- 

 grained, massive quartzite varying to grayish-yellow gray sandstone. 

 The lowest stratum in places is pebbly, grading into a conglomerate 

 at the base. The bedding planes range from a few inches to a few 

 feet apart and faint lines of sedimentation are seen. The rock is 

 jointed by sharply cut planes. Higher up in the formation thin 

 bands of gray-brown mottled and green micaceous shale are found 

 locally, increasing in thickness toward the top. The pebbles in the 

 basal bed vary in character from place to place and consist pre- 

 dominantly of the material derived from the immediately underlying- 

 beds. As mentioned elsewhere, this quartzite is in most places appar- 

 ently conformable to the Algonkian, but there is a slight angular 

 unconformity observable east of Helena, and the Marsh shale is in 

 places cut out, so that the quartzite rests directly upon the Helena 

 limestone. The only fossils recognized are scolithus borings. The 

 total thickness of the Flathead quartzite in the Helena district is 

 300 feet. The formation is easily recognized in the topography of 

 the area, as the resistant nature of the beds causes them to form low 

 foothill ridges, which are prominent on the slopes of Mount Helena, 

 and as the cap reef on the mountain ridge running southward from 

 that peak." (Evidently westward as the Flathead does not extend 

 southward from the peak and the ridge extends west-southwest.) 



Bull. 527, U. S. Geo!. Surv., pp. 88-89. 



Twentieth Ann. Rep., U. S. Geoi. Surv., Pt. 3, pp. 284-287. 



Knopf, Bull. 527, U. S. Geol. Surv., 1913, pp. 89-90. 



Idem. 



