378 C. W. TOM LIN SON 



The main division of the Jefferson dolomite is dominated by a 

 rather uniform succession of fairly massive, fetid, dark-brown 

 dolomites, but includes toward its base a minor variegated sequence 

 of thin members (4 to 8, inclusive), among which are light-colored 

 dolomite, and locally (Labarge Mountain, Blacksmith Fork) sand- 

 stone. The correlation of these minor members on a lithologic 

 basis has been attempted, but cannot be considered very reliable. 

 In no two localities is there the same succession of members 

 throughout the formation, although there is a notable degree of 

 correspondence between the Crandall Creek and Teton River sec- 

 tions, for example. In the Dead Indian Creek, Livingston Peak, 

 Logan, Labarge Mountain, and Blacksmith Fork sections the 

 variegated members are relatively ill-developed, and the out- 

 standing feature of the formation is a nearly uniform sequence, 

 200 feet thick or more, of what has been described above as the 

 most characteristic Jefferson type of dolomites. 



Members 12 to 14 of the Devonian system, above the main 

 body of the Jefferson dolomite, are differentiated only at Blacksmith 

 Fork and at Labarge Mountain, where they have a combined thick- 

 ness exceeding 200 feet. They appear not to be represented in the 

 sections in northwestern Wyoming and southern Montana. This 

 fact, with certain other evidence, is strongly suggestive of an 

 emergent interval between Members 14 and 15. (See discussion of 

 disconformities, p. 132.) 



The upper division of the Jefferson dolomite. — Member 18 is 

 distributed with remarkable uniformity over a very wide area, 

 and has been distinguished in nearly every locality within the scope 

 of this thesis where the Jefferson dolomite has been described in 

 any detail. Its brecciated structure, together with an abundance 

 of calcite geodes in some places, give its weathered surfaces a char- 

 acteristic nodular, pitted appearance. It is everywhere separated 

 from the main mass of the Jefferson by a sequence of thin-bedded, 

 largely platy dolomites. At Labarge Mountain the base of this 

 sequence is marked by 10 feet of sandstone (Member 15); and in 

 the Teton River section its lower portion contains some thin beds 

 of sandstone, and quartz grains are scattered through several of the 

 beds of dolomite. 



