170 GEOLOGICAL SURVEY OF THE TERRITORIES. 



regular faces. There are numerous crystals of sanidine in the obsidian. 

 As we ascend, the bed becomes softer, and besides sanidine contains 

 small spherulitic masses, which are generally about the size of* a pin- 

 head. In some places, however, they are an inch or two in diameter, 

 and when broken open the cavities in them are found coated with 

 Hyalite. This spherulitic obsidian is exactly like that found by us in the 

 Yellowstone National Park,* and a description of one would answer for 

 the other. Layer No. 6, in the section given above, is a compact jaspery- 

 looking rock, slabs of which ring under blows of the hammer. In the 

 cry pto-cry stall ine paste are crystals of sanidine, bronze mica, free quartz 

 in abundance, and occasionally a pebble of what has the appearance of 

 having been tufa inclosed and metamorphosed. This rhyolite is vesicular, 

 the cavities being lined with blue chalcedony. These cavities are most 

 abundant in the lower part. The description of this rock answers for 

 the layer wherever it is shown along the Gunnison, the only difference 

 being in the color, which at station 73 is a purplish-brown. At station 

 77 it is more of a gray. The geodes of chalcedony are very abundant 

 in the latter place. Layer No. 7 breaks into slab-like masses which 

 weather white. They ring under the hammer like the layer below. 

 Just above it are indications of a tufa, resembliug that above the 

 breccia. It seems to be of a reddish color. Along the second creek 

 west of station 73 the trachytic capping has been removed, and the 

 breccia forms the basis for a considerable distance up the creek, 

 the mesa-form disappearing with the removal of the trachyte. The 

 creek west of stations 77 and 78 forms the present western boundary 

 of this trachytic area. At the head of the creek, as seen from station 70, 

 the trachyte is tipped up, dipping toward the south or southeast at an 

 angle of 10° to 15°. This is also the general direction of the slope of the 

 mesas. Station 71) was located on a point capped with a remuant of 

 the lower part of the trachyte, below which is the breccia. It is im- 

 possible to tell at present how far west this flow originally extended. 



The breccia which we have referred to so often in the present chap- 

 ter is geuerally of a dark-gray color in the matrix. The included masses 

 are of all sizes, and generally angular. The greatest variety is seen 

 near station 31. I have already spoken of the stratified character at 

 this point, which seems to indicate its deposition in water. 



The upper layers seem to be lighter-colored and to have the included 

 smaller masses. Farther down there is a dark band, below which the 

 included rocks are in large masses. These layers are variegated, red, 

 green, yellow, and gray. Between the layers are bands resembling a 

 hard sandstone and also tufaceous layers. The included masses are, I 

 think, all trachytic. The tufaceous layers are pink and ash colored, 

 and contain conspicuously black mica and minute crystalsof hornblende. 



Preceding the deposition of this breccia, there was considerable- 

 erosion, as is indicated by comparing the underlying rocks at station 73 

 with those under station 79. In the' former place there are only a few 

 feet of shales between the Dakota group and the bottom of the breccia, 

 while at station 79 there must be at least 1,000 feet, and there is probably 

 more farther back. On the Gunnison, also, as seen by the sections in Plate 

 XIV, there is abundant evidence of such erosion. The draiuage had 

 probably the same general direction as at present. It is also probable 

 that there was an interval between the deposition of the breccia and 

 the flow of the rhyolitic rocks, during which there may have been 

 some erosion of the breccia. It varies greatly in thickness. At station 

 31 it is 3,000 feet, while on the Gunnison, at station 73, it is only 400 feet. 



* Report U. S. Geol. Survey, 1872, page 131. 



