CAMBRIAN AND LOWEE ORDOVICIAN. 129 



In the vicinity of Silver HUl, about 12 miles west of Anaconda, where the best exposures 

 occur, a threefold subdivision of the formation can be made; it here comprises in descending 

 order: 



Feet. 

 Shales, calcareous, strongly banded in brown, white, and green, interbedded with laminated 



limestone 90 



Limestone with thin brown siliceous laminae about 1 inch apart 120 



Shale, dark green, not noticeably calcareous, with a 3-toot intrusive sheet of dark igneous rock 



, near the base 120 



330 



In places, as near Princeton, these rocks are very poorly exposed and here not even a two- 

 fold subdivision can be actually made on the ground. The middle and uppermost divisions 

 can rarely be separated. The thin-bedded rocks between the Flathead and the Hasmark there- 

 fore constitute a convenient stratigraphic unit. 



The Paleozoic rocks above the Silver HUl formation consist preponderantly of thick 

 limestone strata, which are separated by relatively thin shales. The first two of the calcareous 

 strata, with the shale that parts them, are mapped as the Hasmark formation. It comprises 

 a basal member chiefly of blue-gray dolomite, a medial member of dark shales, and a top 

 member of white fine-grained dolomite. The highly magnesian character of the highest and 

 lowest members is apparently not shared by the supposedly equivalent Pilgrim limestone. 

 The lower dolomite is about 600 feet thick; the upper about 350 feet. The thickness of the 

 medial shale member varies widely. Its maximum, attained in the southern part of the quad- 

 rangle, is about 150 fefet; at PhUipsburg it is virtually absent, although a few fragments of 

 shaly rock are locally found at the horizon separating the gray from the white dolomite. On 

 Boulder Creek, also, near the northern boundary of the quadrangle, this horizon is marked by 

 only a few feet of shaly limestone. The shale has yielded a few imperfect shells like Obolus. 



The upper and main part of the Red Lion formation has a very peculiar lithologic char- 

 acter, not shown by the corresponding beds farther east. It consists of gray, drab, and white, 

 rather thickly stratified limestones, whose bedding is marked by closely spaced laminae of 

 siliceous or argUlaceous material, about one-fourth of an inch in average thickness, which 

 project in rehef on the weathered surface. These are invariably wavy or crinkly, and locally, 

 where they constitute the greater part of the rock, they anastomose so that the limestone forms 

 little isolated lenses flattened parallel to the bedding planes." 



The lowest part of this peculiar limestone is the most sfliceous and shaly, and it may 

 roughly be said to grade downward into the lower member, consisting of black shale and thin- 

 bedded magnesian hmestone, without the cherty banding. This member has a thickness of 

 only about 15 feet on the west side of Cable Mountain, but although it is rarely well exposed, 

 it is apparently two or three times as thick in some other sections. 



Fossils were collected from the upper part of the Red Lion formation by Mr. Kindle in 

 1907 and submitted to Secretary Walcott, of the Smithsonian Institution. In a lot from Rock 

 Creek, in the western part of the Anaconda Range, he found Billingsella coloradoensis Shumard 

 and Anomocare sp.; in a lot from the vicinity of Princeton, the same forms together with 

 Cyrtolites sp. and Agraulos sp. AH the specimens, according to Dr. Walcott,* "are of Upper 

 Cambrian age and therefore correspond to the Yogo limestone." 



Correlation of Cambrian formations. — The ascription of the series above described as a 

 whole to the Cambrian depends on (1) the unconformity at the base of the lowest quartzitic 

 member which can reasonably be correlated only with the widespread unconformity at the 

 base of the Flathead, the lowest Cambrian formation of the known Montana sections, and (2) 

 the presence of Upper Cambrian fossils in the limestone of the Red Lion formation. Thus 



« The structure of this rock is very similar to that of the Abrigo limestone, illustrated in "Geology and ore deposits 

 of the Bisbee quadrangle," by F. L. Ransome (Prof. Paper U. S. Geol. Survey No. 21, 1904, PI. VI, B). 

 b Letter to P. C. Calkins, June 3, 1908. 

 48011°— 12 9 



