212 (;. n. walcott — i'iie-cambrian fossilifehous formations 



isli shales before reaching the red Spokane shales, which underlie the 

 Cambrian in the Spokane hills. 



The relations of the Cambrian and the subjacent Belt terrane on the 

 •line of the section from Helena eastward across the Spokane hills to the 

 Big Belt mountains are indicated in the diagrammatic section, figure 2. 



Northwest of Helena the contact between the Cambrian and the Belt 

 teri-ane is followed to tlie crossing of Little Prickly Pear creek, 6 miles 

 west of Marys ville. The Helena limestone outcrops all along the hills 

 and gulches, and at Marysville the subjacent Empire shales occur be- 

 neath the limestone. West of the Marysville Canyon area the silicious 

 beds dip from 10 to 15 degrees to the northwest and pass above into the 

 Helena limestone series, on which rest the Marsh shales. Crossing east- 

 northeast, to the Gates of the Mountain, on the Missouri, 18 miles north 

 of Helena, one finds the Cambrian sandstones resting on the red Spokane 

 shales. This contact is again well shown on the eastern side of the 

 Missouri river, on the road to Beaver creek. On Beaver creek the Cam- 



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CAMBRIA /V 



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Figure 3. — Unconformity between Helena Lime- 

 stone and Cambrian Sandstones. 



The locality indicated is one mile southeast 

 of Helena, Montana. 



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Figure 4. — Unconformity between Marsh Shale 

 and Cambrian Sandstone. 



The locality indicated is two miles southeast 

 of Helena, Blontana. 



brian rests directl}^ on the Spokane shales, which, with the Grayson 

 shales, constitute a thickness of several thousand feet between the New- 

 land limestone and the base of the Cambrian. The contact at the 

 crossing of Soap and Trout creeks, to the northeast, is essentially the 

 same as at Beaver creek and the Spokane hills, although there is a 

 variation in the beds of the Gre3^son, which come in contact with the 

 Cambrian. 



At most of the outcrops where the lower beds of the Flathead (Cam- 

 brian) sandstones come in contact with the Belt rocks the dip and strike 

 of the two are usuallv conformable, so far as can be determined by 

 measurement. Tiiis holds good all around the great Belt Mountain 

 uplift. It is only when the contacts are examined in detail, as near 

 Helena, that the minor unconformities are discovered (figures 3 and 4), 

 and only when comparisons are made between sections at some distance 

 from each other that the extent of the unconformity becomes apparent. 



