502 KEPORT UNITED STATES GEOLOGICAL SURVEY. 



possibly constitute the floor upon whicli the later quaternary detritus of 

 the present basin surface rests 5 but they are accessible for satisfactory 

 study only at comparatively few points, as the majority of the exposures 

 south of Leigh's Creek, on the east side, and Horse Creek, on the west side, 

 are mere remnants, and these much obscured by late weather action. But 

 in the canon of the North Fork of Pierre's Eiver, where it descends the 

 great foreland, these deposits are exhibited in magnificent sections, 

 miles in linear extent, and revealing a vertical thickness of several hun- 

 dred feet. This foreland seems to be made up of several distinct benches 

 of different volcanic iiroducts, which higher on the mountain side de- 

 cline at a rate a little gTcater than the descent of the stream, so that, in 

 passing from the mountain to the plain, successively higher flows are 

 met with. At the head of the North Fork the earliest flowed mate- 

 rial is in contact with Silurian and Carboniferous deposits, the steep 

 bluff' face here showing the lower masses of rusty, coarse-vesicular 

 trachytic lava, with obsidian, which is so like the remnants before noticed 

 in the northern summits of the Caribou Eange that their identity can 

 hardly be questioned. This ledge is underlaid by decomposing porphy- 

 ritic obsidian trachytes having a burned-out appearance in places, whose 

 coarse sand debris litters the steep talus slopes. The obsidian, occurring 

 in fragments here as elsewhere, contains little globules of spherulite ; 

 but these lower beds are so obscured by the action of the weather, which 

 has covered them with a mantle of their own detritus, as to conceal the 

 point of contact with the sedimeutaries as well as the inferior portion, 

 of their own ledges, which more extended search than we were able to 

 bestow will doubtless yet bring to light. But no evidence of the pecu- 

 liar semi-fragmental volcanic deposits, such as apparently constitute the 

 basis of these early volcanic flows in the summit region of the Caribou 

 Mountains, was observed. These deposits once, doubtless, reached high 

 up on the mountain-side iu the vicinity of Station XXXII, but at a 

 period before the North Fork had begun to excavate its canon, in the 

 process of which not only the volcanics, but a vast amount of sedimeu- 

 taries also, were swept away and mingled with the finely-comminuted 

 soil which spreads a mantle of fertility over the undulating uplands 

 which stretch far out into the great plain. South of the North Fork 

 these same volcanics reach well up on the ancient denuded Archaean 

 mountain flank, and still farther in the same direction they exist as mere 

 remnants of the rusty porphyritic and pink and drab trachytes, always 

 inclining toward the i3asin. However advanced erosion had progressed 

 in leveling the great range prior to the effusions of the igneous matter, 

 much of the more conspicuous effects of erosion that strike the observer 

 to-day, such as the gorges which cut the great foreland, have been exe- 

 cuted subsequent to the volcanic emissions. If the canon- valleys reached 

 their x^resent depth prior to the volcanic flow, which seems, very doubt- 

 ful, they have been entirely cleaned out and their beds fifled with the 

 debris swept down from the interior portion of the range. This is appar- 

 ent from an examination of any of the canon mouths south of Bear 

 Creek. 



Descending the foreland either side of the North Fork of Pierre's 

 B-iver, the earlier volcanic flow is succeeded above by successively more 

 recent flows, which present considerable variety in physical appearance. 

 Throughout the canon a heavy mass of laminated perphyritic trachyte 

 prevails, extending to the junction of Pierre's Eiver, and doubtless be- 

 low that point, and which has a peculiar style of weathering, generally 

 in parallel planes, simulating the deposition planes of sedimentary de- 

 posits. Portions, in the upper part of this laminated mass, are, however, 



