278 WINTHROP P. HAYNES 



The following section of the Cambrian northeast of Logan, 

 Montana, was measured by the writer. 



Dr. Peaie's Nomenclature Dr. Weed's Nomenclature Thickness 



i. Pebbly limestone = Yogo limestone 75 feet 



2. Dry Creek shale = Dry Creek shale 20 



3. Mottled limestone = Pilgrim limestone 300 



4. Obolella shale = Park shale 280 



5. Trilobite limestone = Meagher limestone 175 



6. Flathead shale = Wolsey shale AS 0± 



7. Flathead quartzite = Flathead quartzite 200 



Total 1, 500 ^ feet 



Fossils from the Yogo limestone have been submitted by the 

 writer to Dr. Walcott, who considers them of Upper Cambrian 

 age, while those from the Meagher limestone are regarded by him 

 as of Middle Cambrian age. Apparently Lower Cambrian strata 

 are entirely absent in sections in this region. Although the bound- 

 ary between the Middle and Upper Cambrian strata has not been 

 definitely ascertained, it is likely that it comes between members 

 2 and 3. 



Absence of Ordovician and Silurian strata. — -In all of the sections 

 studied by the writer in the Three Forks quadrangle and the 

 neighboring district to the north, the Jefferson limestone lies in 

 apparent conformity on the Yogo limestone without any inter- 

 vening formations. The lower portion of the Jefferson limestone 

 has been considered by Dr. Peale and others as probably of Ordo- 

 vician and Silurian ages, although no fossils of those periods have 

 been found in it. Dr. Kindle 1 has described the Jefferson lime- 

 stone and its fauna and established its age as chiefly Middle Devo- 

 nian with the lower part probably Lower Devonian. 



In one or two good sections studied by the writer some rather 

 poorly preserved corals were found within 25 feet of the base of the 

 formation. These were identified as Favorites cf. limitaris Rom., 

 which is rather common in much of the Jefferson limestone. The 

 presence of these fossil corals is regarded as indicating the Devonian 

 age of all of the Jefferson limestone, and since the gray Yogo lime- 



1 E. M. Kindle, Bull. Amer. Pal. No. 20, 1908. 



