14 Annals of the Carnegie Museum. 



Yellow laminated sandstones 25 ± feet. 



Upper Shales Greenish gray nodular limestones, 



Dark colored argillaceous limestones, 



Soft shaly black and purplish calcareous limestones. . 45 feet. 



Fine green argillaceous shale 30 " 



Intermediate Compact grayish brown limestone, weathers into 



Limestone orange debris and obscures lower shales 15-20 feet. 



Lower Shales Reddish and brownish yellow argillaceous shales .... 50 " 



145 =t feet. 



Dr. Peale noted the absence of fossils in the Lower Shales and the over- 

 lying limestone, and the great abundance of fossils of Devonian age 

 in the Upper Shales, particularly in the calcareous portions. He also 

 noted the presence of a band of black slate or shale in the section at 

 the base of the Yellow Sandstone member at two localities, one near 

 the Horseshoe Bend of the Missouri River near Rekap, and the other 

 south of the Jefferson River near Antelope Creek. 



The writer has visited both of these localities and has included 

 sections measured at both places among the lists of sections given in 

 the following pages. 



Dr. Peale in summarizing his description of the Three Forks Shales 

 noted the fact that they become more arenaceous to the east of Three 

 Forks, as they pass into the Bridger Range, while they become more 

 calcareous to the west on the north side of the Jefferson River. 



This description given by Dr. Peale applies to the Three Forks 

 Formation as seen in the northern part of the Three Forks quadrangle, 

 but in the southern part the formation has changed and becomes more 

 dominantly a limestone with argillaceous and arenaceous phases. 

 Owing to the fact that the strata called the Three Forks Shales by Dr. 

 Peale are a composite series and include limestones and shales and 

 some sandstones, the writer feels that it is advisable to use the name 

 " Three Forks Formation " which is applicable to the southern occur- 

 rences as well as to those about Three Forks. 



The writer has found it possible to divide the Three Forks Formation 

 into seven members, which are easily recognized by their lithologic 

 characters and are present in all of the sections studied in the northern 

 part of the quadrangle and the adjacent region to the north. This 

 sevenfold division will be noted in the case of each section, so that a 



