CARBONIFEROUS UNDIVIDED. 381 



to the Carboniferous. The series is similar to the Delhi division of the Calaveras forma-tion in 

 the Sierra Nevada,"* which is believed to belong to the Carboniferous. 



WeU-defined Carboniferous fossils have been found by Prof. Condon in the Crooked River 

 drainage, in the extreme western part of the Blue Mountains. 



L 12. WESTERN" MONTANA. 



The general succession of Carboniferous rocks in the PhiUpsburg quadrangle, 

 Montana, as stated by F. C. Calkins, in an unpublished manuscript, is as follows : 



Pennsylvanian, Quadrant formation: Feet. 



Quartzitic member 400 



Shaly member 600-50 



Slight unconformity (?) 



Mississippian, Madison limestone: 



Thick-bedded limestone 1,000-300 



Shaly black limestone 300 



Madison liTuestone. — The Madison limestone in all the Montana sections has basal shaly 

 beds overlain by more massive beds. Although in the Little Belt mines Weed has recognized 

 three lithologic subdivisions, only two are recognizable in the Philipsburg quadrangle. The 

 lower consists mainly of flaggy limestones, black on fresh fracture but weathering to a delicate 

 blue-gray or drab tint. In most places two thin beds of coal-black calcareous shale are inter- 

 calated with these limestones, one lying near the base and the other a little below the middle of 

 the lower member. The upper member contains abundant chert. Its lower part is mostly dark 

 blue-gray and in beds of moderate thickness. The upper part is of more massive limestones, 

 mostly white to pale gray and nonmagnesian, their smoothly weathered surfaces and trans- 

 lucence being in contrast to the dull and gritty character of the Jefferson limestone. 



Dr. Girty characterizes the abundant fossils collected from the Madison limestone in the 

 Philipsburg quadrangle as referable to the lower Mississippian fauna, which is so widely 

 distributed over the West. He has identified the following forms: 



Rhipidomella michelini? 

 Chonetes illinoisensis. 

 Productus Isevicosta. 

 Camarotoechia metallica. 

 Spirifer centronatus. 

 Composita immatura. 

 Composita sp. 

 Eumetria marcyi. 



Syringopora surcularia. 

 Syringopora sp. 

 Aulopora geometrica. 

 Menophyllum ulrichanum. 

 Amplexus sp. 

 Echinocrinua sp. 

 Fenestella 2 sp. 

 Schuchertella inflata. 

 Rhipidomella pulchella. 



Apparent unconformity 'between Pennsylvanian and Mississippian. — The difference of age 

 between the faunas from the Madison and Quadrant indicates the lapse of a considerable time 

 interval between them. Physical evidence of unconformity is given by the sharp Uthologic 

 distinction between the uppermost beds of Madison limestone and the lowest part of the Quad- 

 rant formation. A further indication of an erosion interval between them is the dwindling of 

 the upper Madison west of Foster Creek in the Flint Creek Range, to about half its normal 

 thickness. Angular unconformity has not been proved. 



Quodrant formation. — The two members of the Quadrant are Kthologically in strong con- 

 trast. The lower, generally not well exposed except where indurated by contact metamorphism, 

 consists of maroon shales containing gray concretions and thin-bedded gray, white, and pink 

 magnesian limestones. The thickest limestone layer observed is about 8 feet thick. The best- 

 exposed section is on the east fork of Rock Creek in the Anaconda Range. Here 125 feet of 

 shale and limestone immediately beneath the quartzite are continuously exposed, and the 

 imperfect exposure below indicates a total thickness of about 500 feet. This is near the 

 maximum, however, and at the place above mentioned where the Madison was observed in 



a Colfax folio (No. 66), Geol. Atlas U. S., U. S. Geol. Survey, 1900. 



