Annual report of the United States Geological Survey to the Secretary of the Interior . example, the Swauk sandstone dips away from theWenache Mountains at certain localities, and has clearly been upraised,together with the schists, greenstones, serpentine, and granite com-posing that elevation. The basal conglomerate of the Swauk is notonly tilted but in places highly inclined, and the pebbles of which itis composed, as is shown in a number of exposures, have been crushedand faulted. Some of the bowlderdike masses of serpentine in the con-glomerate are smoothed and slickensided, owing to move
Annual report of the United States Geological Survey to the Secretary of the Interior . example, the Swauk sandstone dips away from theWenache Mountains at certain localities, and has clearly been upraised,together with the schists, greenstones, serpentine, and granite com-posing that elevation. The basal conglomerate of the Swauk is notonly tilted but in places highly inclined, and the pebbles of which itis composed, as is shown in a number of exposures, have been crushedand faulted. Some of the bowlderdike masses of serpentine in the con-glomerate are smoothed and slickensided, owing to movements in theirmatrix. These facts lead to the provisional conclusion that there havebeen at least two periods during which the Wenache Mountains haveexperienced an upward movement. This discussion, however, belongsmore properly to the detailed report on the Mount Stuart quadrangle,and will therefore be postponed. The rocks of this formation restuncon-formably on the schists, greenstones, serpentine, forming thefoothills of the Wenache Mountains, and are conformably overlain by. UNIVERSITYILLINOIS. BD83BLL.] SWAIK SANDSTONE. 121 volcanic tuffs and lava Hows, designated in this paper the earliersheet of the Columbia lava. The most marked feature of the region occupied by the Swank sand-stone to the south of the Wenache Mountains is the manner in whichit is traversed by dikes. These are all or almost all of dark, basicrocks, corresponding lithologically with the basalt known as Columbialava, and occupy, in part at least, the fissures through which thatextensive series of lava sheets came to the surface. The Columbialava has been eroded from the area now occupied by the outcrops ofthe Swank where the dikes occur, so as to furnish much information inreference to the manner in which the material forming the vast scriesof lava sheets was forced out. The dikes referred to occur in hundredsand probably in thousands. Practically they are countless. In onesection near the head of St
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