186 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



it below. The rock is of a hard, fine, semi- vitreous base, light gray or 

 greenish-white in color, and inclosing crystals of orthoclase or sanadite. 

 The trend of these two dikes is toward a hill which lies about eleven 

 miles from the Grand, and which, in its isolation and abruptness^ pre- 

 sents a unique topographical feature on the otherwise regularly formed 

 valley, and therefore indicates some equally unique geological fact. The 

 main valley seems to lie between this hill and the Williams Mountains, 

 but the river appears to leave its valley and turn out of its course to 

 cut a deep caiion around the western base of the hill, which rises to a 

 height of 1,500 feet above the stream. Looked at either from up or 

 down the river it shows a series of massive beds, with softer layers be- 

 tween, all dipping eastward, the upper beds at an angle of 30°. These 

 massive beds are of trachyte; the softer beds are Cretaceous shales, and 

 the bedding is apparently so perfect as to give the impression that the 

 lava was contemj)oraneous with the sedimentary rocks, and that here 

 were trachytes of Cretaceous age. But either north or south of the 

 hill the Cretaceous shales in the neighboring ravines show very flat 

 eastward dips, indicating that the lavas are but intrusive masses of 

 post-Cretaceous age, which, instead of breaking across the strata, here 

 followed along their planes of bedding, and forcing apart and upward 

 the strata between which they wedged themselves, caused them to 

 incline eastward at a steeper angle than those on either side. Ex- 

 amination of the hill confirms this idea. In a single section made 

 about in the middle of the hill several points were found where the lava 

 broke abruptly across one or two feet of the shales, breaks of a few 

 inches being common, while a generally uneven surface exists between 

 the two. At one point a limestone rested on a lava tor a little ways, 

 and then slates commenced to wedge in between the two, generally with 

 broken edges. The section is indicated in Plate III, section 5, and is 

 given in more detail below. The three thick lower beds of trachyte in- 

 dicated in the section are all joined in one on the southwest corner of 

 the hill, and form a high i^innacled cliff overhanging the river. Near 

 here, and a little south, may be seen what appears to be the side or edge 

 of a flow, where the undisturbed slates lie on the south, dipping at 

 an angle of 10° eastward, and abutting against lava which from there 

 north forms a layer, and on which rest the slates which have been turned 

 up by the lava. On the opposite (west) side of the river is a massive 

 hill, also apparently of the trachyte, a remnant of the thickening dike, 

 with the capping slates eroded away. On the hill-slopes to the south- 

 west appear some disturbances of the Cretaceous beds, possibly caused 

 by the incoming of the lava. In the river canon about 80 feet of a white 

 siliceous sandstone is exposed, probably the upper bed of the Lower 

 Cretaceous sandstones No. 1. It dips but a few degrees to the east. 

 The beds included between the trachytes gradually increase their dip in 

 ascending the hill. They are mostly of dark argillaceous shales, with 

 some blue-black slates, and one or two limestones. The usual Cre- 

 taceous fossils occur here and there. Considering the preponderance 

 of lava in the hill the sedimentary rocks seem to have been but very 

 little affected by any heat that may have accompanied its eruption. 

 What little effect it has produced, however, is as markedly on the beds 

 lying above lava as on those below it. The lava throughout is a hand- 

 some trachyte, with a tendency to very large feldspar crystals, which 

 are inclined to glassy, and seem to be of the sanadite variety of ortho- 

 clase, though the usual orthoclase forms are more common than the 

 square tabular crystals. Some of the latter are from one to two inches 

 across. The upper trachytes seemed usually more porphyritic than the 



