Haynes: Fauna of the Upper Devonian in Montana. 21 



A comparison of all of the sections shows a distinct increase to the 

 southwest in the amount of limestone in the formation. This is due 

 chiefly to the increase in thickness of member 6 and the predominance 

 of limestone in member 7. Thus there is a gradation toward the con- 

 ditions which prevail in the southern part of the quadrangle. 



Fossils were obtained from the upper part of the formation at all of 

 the localities where sections were measured. The fossiliferous mem- 

 bers of the formation are numbers i, 2, 4, and 5. Fossils are particu- 

 larly abundant in number 4, the gray limestone, and number 5, the 

 green shale. The fossils in numbers 4 and 5 indicate an Upper De- 

 vonian age, and those in i and 2 indicate a transition into the Missis- 

 sippian. The evidence for the age of the formation will be given 

 with the description of the fossils. 



The sections of the Three Forks Formation studied by the writer in 

 the southern part of the Three Forks quadrangle are not as satisfactory 

 as those just given on account of the much poorer exposures. The 

 country here is much more mountainous, and the shaly beds are 

 obscured by talus and vegetation. No fossils were obtained from the 

 formation here, but this was probably due to insufficient search and 

 poor exposures, because fossils have been found in small numbers in 

 the formation in the northwest corner of the Yellowstone Park, 

 which adjoins the Three Forks quadrangle on the southeast. 



The best section measured by the writer in the southern part of the 

 Three Forks quadrangle is located in the upper end of the West 

 Gallatin Canyon where it opens out into the Lower Basin. Here the 

 strata are downfaulted against the gneiss along a nearly vertical fault 

 plane. The strata strike about N. 40° W., nearly parallel with the 

 fault, and dip about 50° S. The thicknesses of the beds considered to 

 belong to the Three Forks Formation are as follows: 



Base of gray Madison limestone. 



Gray shaly limestone weathering buff, in lower part red and 



yellow 125 =fc feet. 



Brown limestone, breaking into small joint blocks 25 



Yellowish red shaly limestone 40 



Grayish brown limestone breccia 15 " 



Obscured by talus 35 



Total 240 ± feet. 



Brown Jefferson limestone with Favosites cf. limilaris. 



It will be noticed that although the seven members which compose 

 the formation in the northern part of the quadrangle cannot be re- 



