Haynes: Fauna of the Upper Devonian in Montana. 15 



comparison of the thickness of any of the members in ditlercnt parts 

 of the region can easily be made by referring to the numbers. It is 

 noticeable that in all of these sections the thickness of the formation 

 is considerably greater than in the section given by Dr. Peale, which 

 has been generally taken as the standard for the Three Forks Formation. 



For purposes of general comparative study the five columnar sec- 

 tions on Plate IV have been drawn. These sections are arranged in 

 order from left to right, from the northeastern part of the region to the 

 southwestern. 



Throughout all of the region where the Three Forks Formation has 

 been recognized it is almost invariably exposed in a small valley. This 

 is due to the fact that it is prevailingly a shaly formation occurring 

 between two massively bedded limestone formations, and difl'erential 

 erosion has produced the characteristic valley between the Jefferson 

 and Madison limestone ridges. Because the formation generally 

 occurs in a valley the exposures are obscured and the contacts masked 

 by talus and vegetation. Often the valleys were traversed by the 

 writer for several miles without finding any place where a satisfactory 

 section could be measured. In consequence of this many of the sec- 

 tions are incomplete, especially in the lower part, but since no fossils 

 have been found in the two lower members it is not so important that 

 they should be included in the sections. The figures which are given 

 were obtained by measurements with a steel-wire tape and the thick- 

 nesses w'ere either measured directly or computed from the horizontal', 

 distance and the angle of dip. 



The details of the various sections of the Three Forks Formation- 

 studied by the writer will now be considered in order from north ta 

 south. 



Section ^1.— The most northerly occurrence of the formation was 

 observed about four miles east of Lombard and one and one-half 

 miles northeast of Crane, a flag station on the Chicago, Milwaukee, 

 and Puget Sound Railway. Here there is a good exposure in a saddle, 

 back of a cliff of the massive brown Jefferson limestone. The strata 

 have a strike of about N. 70° E. and dip 15° north. The Lower 

 shale member is almost entirely obscured by vegetation and only a 

 small amount of reddish yellow angular shaly fragments in some gullies, 

 indicates its presence. The intermediate limestone member is of a 

 rather bright yellow color and contains some white calcite veins. 

 Above the limestone, which is about fifteen feet thick, there is a good 



