THE BASIN PROVINCE 135 



Calkins and Emmons continue: 



"The kinds of rocks are the same in the PhiHpsburg section as in the typical 

 Quadrant of the Threeforks and Yellowstone National Park region, although 

 the sharp division into a quartzite and a shaly member does not there seem 

 possible. The assignment of a Carboniferous and probably Pennsylvanian age 

 to the upper quartzitic stratum is based on lithologic rather than on paleontologic 

 grounds, for no fossils have been found in it. The upper quartzite was included 

 in the formation primarily because of its resemblance to the lower. Support is 

 lent to this part of the correlation, however, by the opinion of Hoyt S. Gale, 

 who in 1910 examined a section near Melrose, Montana, that is essentially similar 

 to that of the Philipsburg quadrangle. Mr. Gale considers the lower and purer 

 quartzite equivalent to the Weber quartzite of Utah, and the higher beds here 

 included in the Quadrant as equivalent to the Park City formation of Utah. 

 One reason for this correlation is lithologic resemblance, but a stronger one is the 

 occurrence of a phosphate bed in the Melrose section corresponding to one in 

 the Utah and southern Idaho sections. This phosphate lies between the two 

 quartzitic strata and has been found at this horizon on Flagstaff Hill since the 

 geologic survey of the quadrangle was made. The lower shaly member may have 

 its equivalent in the Morgan formation on Utah or in similar rocks found locally 

 in the base of the Weber quartzite." 



Under the description of the geologic history of the region, Calkins and 

 Emmons remark upon the Upper Pennsylvanian: 



"The quartzites of the upper part of the Quadrant are probably beach deposits, 

 superposed on the fine-grained rocks of the lower member after an interval of 

 erosion, for continuous deposition would have been recorded by a gradual instead 

 of an absolutely abrupt Hthological transition. It therefore may be supposed 

 that the inland sea of early Qu^idrant time was filled or upheaved and its bed, 

 after a brief period of erosion, again invaded by the sea, whose advancing margin 

 gradually covered the surface with a layer of beach sands. An interlude in these 

 conditions is represented by the calcareous and phosphatic beds between the 

 quartzite strata. The carbonate and the oolitic phosphate of lime are presumably 

 chemical precipitates, and are most likely to have been formed in a shallow 

 inclosed sea. The wide expanse and the unbroken continuity of the phosphate 

 beds in this region indicates that the sea extended continuously over a large 

 part of Montana, Idaho, Utah, and Wyoming." 



In going directly north from the Yellowstone Park region, the type 

 locality of the Quadrant formation, into Montana, the change in the deposits 

 of the horizon are more abrupt than in going toward the northwest. This is 

 probably due to the northwestward trend of Rocky Mountains at this point. 



In describing the horizon north of the Little Belt Mountains, Weed 

 writes as follows:^ 



" Quadrant formalion. — This formation, named from its prominence in Quad- 

 rant Mountain in the Yellowstone Park, varies in character and increases in 

 thickness from the southern exposure in the canyon of Sixteenmile Creek north- 

 ward. The southern areas of this quadrangle show a series of beds 230 feet thick. 



'Weed, VV. H., Little Belt Mountains Folio, No. 56, U. S. Geological Survey, p. 2, 1899. 



