NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 2/1 



Meagher Limestone 



Thin-bedded, bluish-gray limestone with fossils at base. Strike 



north 50° west. Dip 30° southwest 6 feet. 



Ptychoparia 



Acrotreta 



Iphidea 



Eruptive 11 feet. 



Limestone. At 165 feet from its base the limestone becomes 

 more massive and gray in color, and is made up of thin layers 

 grouped in massive layers. At 360 feet thicker individual layers 

 appear and continue to the top of the formation. Fragments of 

 trilobites show here and there but very rarely. 



Total 720 feet. 



A bed of irregularly-bedded, eruptive rock rests on the limestone 

 beneath the Park shales. 

 Park Shales 



Green and purple argillaceous shale 290 feet. 



Pilgrim Limestone 



(a) Massive-bedded, gray, oolitic limestone, passing above to 



bluish-gray, thin-bedded, fossiliferous limestone 205 feet. 



Fragments of Cambrian fossils were seen in a few layers. 

 {h) Light-gray, arenaceous, finely granular or subcrystalline 

 limestone. (Strike north 40° west. Dip 23° southwest.) 



In the lower 25 feet small Hyolithes occur with bits of trilobites. 



Above, the strata become more massive and coarser 135 feet. 



A bed of intrusive lava 3 feet thick occurs near the base of (b). 

 The Dry Creek shale and Yogo limestone of Weed were not 

 recognized in this section. The shale is usually only a thin bed 

 that is readily concealed by debris from the limestones above. 

 Summary: 



Pilgrim limestone 340 feet. 



Park shale 290 " 



Meagher limestone 720 " 



Wolsey shale 695 " 



Flathead sandstones 640 " 



Total Cambrian 2,685 feet. 



The next overlying limestone is referred to the Devonian. No 

 line of unconformity was noted in the ridge where the section was 

 measured, although there is undoubtedly an unconformity by non- 

 deposition of the Ordovician and .Silurian. 

 Devonian 



la. Massive-bedded, dark steel-gray, arenaceous limestone, 

 weathering to a dirty brownish-gray (oil-stain brown) color. 



Obscure fragments of fossils occur at the base. At 65 feet a 

 band 18 inches thick in a massive layer 3 feet thick is almost made 

 up of Stromatopora, Faz'osites, etc. 



