162 THE CAMBRIAN. [bull. 81. 



On the map accompanying the twelfth annual report of the Hay den 

 survey, issued in 1883, the geographic distribu tion of the strata re- 

 ferred to the Silurian (which includes the Cambrian and the quartzite 

 referred to the Potsdam and a portion of the limestones referred to the 

 Quebec) is delineated as it occurs in southeastern Idaho. This is based 

 on the work of Messrs. Bradley, Eudlich, St. John, and Peale, of the 

 Hayden survey. 



MONTANA. 



During the field season of 1872 Dr. F. V. H ayden made a geologic 

 recounaissance of certain portions of Montana and noted the presence 

 of limestone on the Gallatin River, above Gallatin City, that contained 

 numerous fossils. The thickness of the entire series of strata referred 

 to the Lower Silurian is estimated at 1,600 feet. The massive lime- 

 stones referred to the Potsdam group are about 400 feet thick and 

 pass gradually down into 50 feet of thinly ]aminated,.cherty limestones 

 or calcareous mud layers, with abundant organic remains. These are 

 separated by a purplish sa ndstone that rests upon variegated shaly 

 clays, yellow, green, etc., then hard, dull purplish brown quartzose 

 sandstones, which in turn are underlaid by alternating sandstones 

 and clays, the whole having a thickness of over 1,000 feet. 1 



The fossils collected were studied by Mr. F. B. Meek, who identified | 

 the genera Lingulepis, Conocoryphe, Bathyurus, Acrotreta, Hyolithes, 

 and Agnostus from the beds of the east side of the Gallatin River. 2 



Strata of " Primordial age" were discovered by Messrs. Dana and 

 Grinnell upon the southwestern slope of Little Belt Mountains and the 

 south extremity of the Belt range of mountains; these are referred to 

 with certainty, owing to the finding of Primordial fossils. 3 



In 1884 Dr. F. V. Hayden revisited the Gallatin River locality, in 

 company with Dr. A. O. Peale. 4 The section near the mouth of the 

 East Gallatin River was studied in detail by Dr. Peale and two litho- 

 logically well defined groups determined. 



In the report of the following year the East Gallatin group is spoken 

 of as probably Middle Cambrian. 5 



In his administrative report to the Director of the U. S. Geological 

 Survey for the field season of 1889, Dr. A. O. Peale tabulates the Gal- 

 latin River Paleozoic section, and includes under the Cambrian the 

 Gallatin limestone, 835 feet in thickness, and the Gallatin sandstone,- 

 415 feet. Beneath the Gallatin limestones are 5,000 feet of conglomer- 



J Report of F. V. Hayden, TJ. S. G-eol. Sur. of the Terr., 6th Ann. Rep., 1873. p. 72. 



'Preliminary paleontological report, . . . with remarks on the ages of the rocks, etc. U.S. 

 Geol. Sur. of the Territories, embracing portions of Montana, Idaho, Wyoming, and Utah; 6th Ann. 

 Rep., 1873, pp., 431,432. 



3 Geological report. Report of a geological reconnoissance from Carroll, Montana Terrritory, on 

 the Upper Missouri, to the Yellowstone Park and return, made in the summer of 1875, by Wm. Lud- 

 low; 1876, p. 22. 



4 Administrative Report Montana Division. U. S. Geol. Survey, 6th Ann. Rep., 1884-'85, 1885, pp. 49- 

 51. 



•Ibid., U. S. Geol. Survey, 7th Ann. Rep. for 1885-'86, 1888, p. 86. 



