128 INDEX TO THE STRATIGEAPHY OF NOETH AMERICA. 



The Middle Cambrian series is well developed and somewhat diversified around 

 the Little Belt Mountains. Weed *•*" distinguished the following: 



Middle Cambrian: Feet. 



Yogo limestone 70 



Dry Creek shale 40 



Pilgrim limestone 80 



Paris: shale 800 



Meagher limestone 60 



Wolsey shale 150 



Flathead sandstone 60 



With reference to the above statements, Ulrich comments (personal note) : 



The series of rocks in Montana and Wyoming from the base of the Flathead quartzite to 

 the top of the Gallatin limestone seems essentially equivalent to the Cambrian sections in central 

 Texas, Oklahoma, Missouri, and Iowa. In each case we have a good Upper Cambrian sandstone 

 followed, except in the far Northwest, by younger sandstone and dolomites. As to the base of 

 the Cambrian, it is a question if at any of these localities any Middle Cambrian (Uke that in 

 the Great Basin or hke the Acadian of the East) is present. 



The following description of the Cambrian formations in the Philipsburg quad- 

 rangle, Montana, and the discussion of their correlation are condensed from the 

 description in an unpublished paper on the geology and ore deposits of that quad- 

 rangle, by F. C. Calkins, and from the unpublished folio on the quadrangle, by the 

 same author. 



The Flathead quartzite is composed of more or less vitreous quartzite with an average 

 thickness of about 200 feet. It lies unconformiably upon pre-Cambrian red shales and sand- 

 stones equivalent to the Spokane shale of the typical Belt section. The evidence of this uncon- 

 formity, whose importance was long ago estabhshed by Walcott, is especially clear in the Philips- 

 burg district. Near the head of Rock Creek, in the Anaconda Range, a chfl exposure shows the 

 base of the Flathead resting on the Spokane with an angular discordance of about 30°. On a 

 branch of Lost Creek, about 8 miles northwest of Anaconda, a shghter discordance is displayed, 

 and here the base of the Flathead is locally a coarse conglomerate of well-rounded pebbles of a 

 quartzitic sandstone identical with that which forms the immediately underlying Spokane beds. 

 Due probably to beveling in the erosion interval preceding the deposition of the Flathead are the 

 great and rapid variations in the thickness of the Spokane formation. This is at least 5,000 feet 

 thick in the gorge north of Georgetown Lake, while at the head ofMiU Creek, 13 miles southeast, 

 the Flathead is separated from the Newland limestone by only a few scores of feet of the Spo- 

 kane formation. There is hke wise a great diminution in the thickness of the Spokane between 

 the northwest corner of the quadrangle and the center of its northern margin. The uphft that 

 initiated the beveling seems therefore to have been relatively greater to the eastward. 



There is some evidence that a shore line of the Flathead sea traversed the southeast part 

 of the quadrangle. In the locality near the head of Mill Creek where the Spokane is thinnest, 

 the Flathead quartzite is irregular in thickness, much thinner on the whole than in most other 

 places, and at one point was seen to taper to a knife-edge, leaving the superjacent Silver Hill 

 formation in direct contact with the Spokane. The locahty is, however, one of complex folding 

 and thrust faulting, and the possibihty of the thinning out of the quartzite by faulting nearly 

 parallel to stratification planes is not absolutely excluded. 



The Silver HjU formation is here on the average about 400 feet thick. A maximum thick- 

 ness of 700 feet was observed near the head of Rock Creek, in the Anaconda Range, but this 

 dwindles to about 150 feet within 10 miles eastward. Its lower part consists mainly of dark- 

 greenish, rather siliceous shales, the upper part of calcareous shales and thin-bedded limestones. 

 It shows rapid gradation into the Flathead quartzite below and the dolomites above. 



