THE BASIN PROVINCE 133 



"After the deposition of the Madison limestones there seems to have been a 

 marked change in the character of the sediments, due possibly to the prevalence 

 of much shallower seas or to more active erosion of land areas. The lower beds, 

 to which the name of ' red ' limestone has been given, are everywhere arenaceous 

 and argillaceous, and in many localities a conglomerate of limestone pebbles lies 

 at the very base of the formation. Although no true dolomites are found, these 

 lower limestones are all more or less magnesian. The section varies considerably 

 at different points, but its thickness is between 300 and 400 feet. The red lime- 

 stone is from 170 to 200 feet thick, and the brilliant red color of its debris makes 

 it very conspicuous, while its soft character leads to the development of a ravine 

 back of the outcrops of the Madison limestone. The fossils found in the red 

 limestone are of upper Carboniferous age. Following the red limestone are from 

 150 to 180 feet of thin-bedded, cherty limestone, alternating with quartzitic 

 layers, the latter predominating at the top and being capped by a prominent 

 bed of quartzite or quartizitic sandstone, which has been taken as the base of 

 the overlying Mesozoic." 



At Old Baldy, near Virginia City, the limestone is capped by the Quad- 

 rant quartzite. Immediately below the quartzite there is a considerable 

 thickness of limestone which contains Pennsylvanian fossils. Schuchert 

 considered these as Pottsvillian (Bull. Geol. Soc. Amer., vol. 20, p. 559, 

 1910). The Quadrant sandstone is certainly calcareous at its base, and 

 also fossiliferous. 



In the description by Clark^ of the rocks of southwestern Montana the 

 position of the Quadrant is given as well up in the Pennsylvanian in opposi- 

 tion to the opinion expressed by Girty,^ who regards it as Mississippian, 

 in some places at least. 



Near Melrose, Montana, which lies about 27 miles directly south of 

 Butte, the Quadrant lies in almost the same position and relations as the 

 Weber occupies in southeastern Idaho and the adjacent portions of Utah 

 and Wyoming. Gale says:^ 



"The quartzite of the Quadrant is overlain by light, sandy-weathering blue 

 limestone, about 130 feet thick in the Melrose section. This limestone contains 

 much black chert in nodular form and in layers. The phosphate bed immediately 

 overlies this sandy blue limestone and is itself overlain by ledges containing much 

 massive chert, so that the stratigraphic section here corresponds remarkably in 

 lithologic character with the sequence of strata typically associated with the 

 phosphate beds in southeastern Idaho." 



Near Dell and Dillon, Montana, Bowen^ reports the following sequence: 



"Overlying the limestone (No. 2) (of Mississippian age) is a great thickness of 

 sandstone containing some highly calcareous beds and possibly some true lime- 



' Clark, T. H., Bull. Mus. Comp. Zool. Harvard University, vol. Lxi, No. 9, p. 361, 1917. 



2 Comment on manuscript of Professional Paper No. 71, p. 387. 



'Gale, Hoyt S., Rock Phosphate Near Melrose, Montana, U. S. Geological Survey, Bull. 



470, p. 444, 1910. 

 * Bowen, C. F., Phosphatic Oil Shales near Dell and Dillon, Beaverhead County, Montana, 



U. S. Geological Survey, Bull. 661, p. 316, 1918. 



