174 GEOLOGICAL SURVEY OF THE TERRITORIES. 



irregular. The highest points in these areas hardly mark the original 

 surface of the flow. The subsequent denudation has been enormous, as 

 will be readily conceived when we compare the elevations of these 

 points with those of the deepest valleys near them. Stations 45 and 

 48 are almost on a line with each other running north and south. 

 This line also passes through our camp ^o. 45 which was on Plateau 

 Creek. Comparing the elevations of these three points we find that the 

 creek at camp 45 is 3,891 feet below station 48, and 3,870 feet below 

 station 45. Farther down, the distance from the north end of the mesa 

 to the level of the creek is over 4,000 feet. The south end of the mesa 

 is 4,077 feet above the level of the Gunnison at a point due south of it. 

 These figures will give some idea of the immense amount of material 

 that has been remove'd since the flow of lava took place, and show also 

 how improbable it is that the original surface still remains. 



The two plateaus that we have just described are evidently the remains 

 of what was once one continuous mass. The elevation of station 48 is 

 practically the same as that of station 45. On comparing the rocks 

 from the two localities, we find that they are identical in appearance 

 and in structure. We have also seen that there is the same slope to 

 the west on both. If they had not been connected we would find traces 

 of the flow in some of the valleys, but we search in vain for any such 

 evidence. ]S"o basalt is seen in the valleys save the bowlders that have 

 been carried down from the plateaus. What the original limits were it 

 is impossible to tell. 



It is difhcult to get at the exact thickness of the basaltic flow as 

 originally spread out. At present it is probably considerably less. 

 At station 48 the total thickness is 248 feet. This is the highest point 

 on that plateau but it hardly represents the original thickness. The 

 thickness at the north end of the mesa is 100 feet. The latter jnay not 

 be absolutely correct. It was determined from station 57 by angles to 

 the top and base of the abrupt wall of the mesa. The actual base 

 may be covered with the talus so that it is concealed. As to the age 

 and the source of the basalt we cannot say anything very definite. It 

 is, however, in all probability comparatively recent, althongh older than 

 the flow mentioned as occurring near the mouth of Eagle Kiver. It 

 rests on sandstones and shales of Tertiary age, which were but little 

 alfected by erosion previous to its pouring out. I was unable to fix on 

 any point as its source ; I think, however, that it lies farther to the 

 north. The surface of the plateaus is covered with good grass, and 

 groves of cottonwoods and pines, among which there are numerous 

 beautiful little lakes. Both of them are very well watered. Near 

 station 45 are three beautiful lakes, two of which head streams flow- 

 ing to Plateau Creek, and one heading a branch of the North Fork of 

 the Gunnison. Near station 45 there is a beautiful illustration of the 

 effects of cooling, in the hexagonal columns into which the mass has 

 separated. They are very regular, and are five or six feet in diameter. 

 The surface of the mass inclines to the northward. 



The edge of the mesa portion of the plateau is very abrupt, especially 

 on the western side, and at the extreme northern and southern ends. 

 It stands out like the wall of a fortress. The outline is irregular. The 

 accompanying map will give a good idea of it as also the areas covered 

 by the basalt. Fig. 3, Plate IX, shows a section made from the edge 

 of the mesa to the Gunnison. On the line A B of map E, a represents 

 the capping of basalt. In all the valleys of the streams heading 

 in the mesa there is an abundance of bowlders of basalt. They are so 

 numerous in some places, even on the long sloping spurs running from 

 it, that it is difficult to determir^e the underlying rock. 



