276 WINTHROP P. HAYNES 



There is still much disagreement among the various geologists 

 who have worked in the parts of the Cordillera where the Belt 

 Series is exposed, in regard to the age of the series and the corre- 

 lation of the different formations in it. The writer is disposed to 

 agree with the correlation in a recent report on the Philipsburg 

 quadrangle in Montana, 1 in which strong evidence is shown for 

 a rather long erosion period between the Belt Series and the over- 

 lying Cambrian quartzite. The two formations identified by the 

 writer as the Empire and Spokane formations are therefore con- 

 sidered to be of Pre-Cambrian, Algonkian, or Proterozoic age. 



PALEOZOIC 



The Paleozoic formations recognized by the writer in this region 

 are for the most part continuous with those described by Dr. 

 Peale in his report on the ''Paleozoic Section in the Vicinity of 

 Three Forks, Montana," 2 and later in the Three Forks Atlas Folio. 3 

 His descriptions of the formations are very good and apply equally 

 well to the exposures in the region to the north, on the Fort Logan 

 Sheet. There are some additional facts concerning the thicknesses 

 and ages of the formations and a few changes in the nomenclature 

 which will be discussed under the following headings: 



Cambrian. — A comparison of sections made by different geolo- 

 gists in the neighboring quadrangles shows that the seven lithologic 

 divisions noted by Dr. Peale in the Three Forks quadrangle are 

 persistent throughout southwestern Montana and the neighboring 

 part of Wyoming. It seems advisable to have but one name for 

 each of these divisions, and since locality names are preferable to 

 descriptive names the writer suggests that the nomenclature used 

 by Dr. Weed 4 in the Little Belt Mountains Folio be adopted for the 

 Cambrian throughout the whole region where these seven lithologic 

 divisions are recognized. 



For purposes of mapping it seems best to keep the broader 

 divisions used by Dr. Peale, the two lower members forming the 

 Flathead formation and the upper five the Gallatin formation. 



I Prof. Paper 78. U.S. Geol. Survey. - Bull. U.S. Geol. Survey, No. no. 



3 Atlas Folio, U. S. Geol. Survey. Xo. 24. 



4 Atlas Folio, ibid., Xo. 56. 



