HOLMES.] THE GRAND CANON. 35 



bum are a number of spurs, formed of coarse basaltic breccias and con- 

 glomerates. The bedding is very irregular and there is a general incli- 

 nation from the mountains of from 3° to lOo. The spur down which the 

 trail passes is underlaid by the strata which constitute the interior parts 

 of the mountain, as attested by a number of outcropping, upturned 

 edges. In the loose material covering the slopes there are many frag- 

 ments of sandstone and quartzite that are doubtless due to the l^reak- 

 ing up of Tertiary strata. The valley of Tower Creek, as before re- 

 marked, is surrounded on three sides by the low ridge of the Washburn 

 Range. The erosion over the greater part of the valley has been car- 

 ried down to a general level corresponding to that of the rhyolite plateau, 

 so that the observer gets the impression that the range is a connected 

 series of cones built up subsequently to the formation of the plateau, by 

 an accumulation of volcanic ejecta. 



I am inclined to the opinion, however, that the range existed prior to 

 the flow of the plateau rocks, as we have in no case found fragments of 

 the plateau rocks included in the steeply inclined strata of the mount- 

 ains, which is also true of the conglomerates. This could hardly be the 

 case if the volcanic channels had penetrated the deposits of the plateau. 

 On the other hand, if this range had existed prior to the rhyolite out- 

 flows, the inolten materials must have been remarkably plastic to have 

 filled in all the indentations on all sides so uniformly and completely. 

 The chief difficulty, however, in determining the question lies in thefaet 

 that the breccias, which are surely subsequent to both the rhyolites and 

 the hornblendic rocks of the mountains, lie upon the flanks of the mount- 

 ains and obscure the junctions. The main channels of the stream have 

 cut down into the plateau formations and flow at the bottoms of pretty 

 deep canons. The surface upon which the conglomerates and breccias 

 were deposited seems to have been pretty uniform, Avith a slight inclina-\ 

 tion toward the north, and down toward the river, in the vicinity of 

 Baronett's Bridge — has been pretty deeply eroded. 



On the northern slope of the northern wing of the mountains, between 

 Black-tail-deer and Elk Creeks, the conglomerates and breccias extend 

 far down toward the third canon of the Yellowstone. These formations 

 are chiefly basaltic and seem to be associated with the numerous basaltic 

 flows that occur in the same district. It is i)robable that the basalts 

 and basaltic breccias were contemporaneous in their origin, and that 

 their formation has continued down to a comparatively recent dat«. 

 The location of the vents from which the lava issued cannot be made 

 out, and as to the origin of the fragmental deposits, I do not wish to 

 advance any theories. The bulk of those about Washburn have been 

 formed of material derived from the mountain itself, and have more the 

 appearance of sub-oerial deposits than any other in this part of the Park. 

 Ill Plate XVII, I present a section which shows the relations of the vari- 

 ous formations of the region to each other. 



GEAND CANON. 



From the summit of Mount Washburn we have the only comprehen- 

 sive view of the Grand Caiion that can be obtained. The apparently 

 bottomless gorge may be traced from its head at the Great Falls to 

 Junction Valley, a distance of nearly 20 miles. From the falls to the 

 base of Washburn Pange it has a pretty direct northeast course. Below 

 the Washburn Gap it curves slightly to the northwest and afterwards 

 turns to the north. From the mouth of Agate Crook to Tower Creek 

 the course is nearly northwest. I si)eak of the passage of the Washburn 



