PEALE.J GEOLOGY — BASALTS — PLATEAU CREEK. 173 



3. Vesicular basalt. Mostly red in color on weathered surfaces. 

 Some of the pieces found at the base are black, and have cavities lined 

 ■with carbonate of lime. 



4. Tufaceous layer, of which only the upper portion could be seen. 

 The color is white, and in some places the layer has masses of basalt. 



In layer aSTo. 1 1 found the following minerals, besides those mentioned 

 above : Hyalite and quartz in segregations and pyrites coating the 

 weathered sorface in a few instances. All these basaltic rocks have 

 magnetite. The local attraction on the station was considerable. 



The cone on which station 48 was located is situated in an isolated 

 patch of basalt of the same character (see map 3). There are several 

 other areas from which i)ointed and coffin-like masses rise, none, how- 

 ever, reaching the altitude of station 48. The basalt, as has been mea- 

 tioned in a previous chapter, rests on dark-gray shales which weather 

 to a chalky whiteness. This is beautifully shown on the north side of 

 the Grand. It was impossible to tell from the station how extensive 

 the area covered by the basalt is to the north. On this plateau the 

 area does not exceed eighteen or twenty miles. To the westward it 

 reaches its limits at least five miles east of station 50. West of the 

 basaltic line the plateau becomes broken: the capping having been re- 

 moved, the soft beds beneath yielded readily to the eroding influences, 

 and, therefore, instead of a plateau there is a sharp ridge, gradually de- 

 creasing in elevation to the westward. The course of the range, for it 

 forms a very well-defined range, is generally west. It is very irregular, 

 however, and the streams on either side cut profoundly in|;o the strata. 



Station 48 has an elevation of 11,063 feet above sea-level. Two and 

 a half miles west the top of the basalt is 150 lower, and two miles farther, 

 that is, four and a half miles west of station 48, it is 350 feet below it. 

 So we see there is a slope to the westward at present. We cannot tell 

 whether this is the original surface, or whether it has been modified by 

 subsequent erosion. It is probable, however, that the original slope 

 was to the westward. We see the same decrease in elevation from east 

 to west on the plateau between the North Fork of the Gunnison and 

 Plateau Creek. On station 43, which is one of the most eastern of the 

 basaltic points on this plateau, the elevation is 11,134 feet. At station 

 44, one mile farther west, it is 11,128, while on station 45, eight and a 

 half miles west of station 43, it is only 10,954, a decrease of 180 feet. 

 Five miles west of station 45 the elevation is 10,904, which is practically 

 the same as that of station 45. Sixteen miles west of this point, at the 

 north end of the mesa, the elevation is only 9,800 feet, and at the south 

 end, which is three and a half miles farther east and sixteen miles south 

 of the north end, it is 9,733 feet. This is a fall of a little over 1,400 feet 

 from station 43, in a distance of about twenty-seven miles. The greater 

 part of the decrease in elevation is in the western portion ; that is, in 

 the last twelve miles. As is evident, on glancing at the figures given 

 above, the eastern portion varies but little on comparing the higher 

 points with each other. About stations 43, 44, and 45 the basalt forms 

 points that rise considerably above the general level, while the sur- 

 rounding country is very much broken up. To the westward, however, 

 the basalt forms a mesa-like capping to the country. This mesa is 

 somewhat irregular in outline, forming at first a narrow strip which 

 divides into two arms, one extending to the southwest and the other to 

 the northwest. They are separated by a small creek that drains into 

 the GuuDison. It has cut gradually deeper and deeper until the basaltic 

 capping has been removed, leaving a tongue-like process of Tertiary 

 rocks between the arms. 



In the eastern part of the divide the areas covered with basalt are 



