208 GEOLOGICAL SURVEY OF THE TERRITORIES. 



more to the westward, the eruptive rocks spread over the highest por- 

 tions of two parallel spurs, connected by a third one. Similar to the 

 rocks of neighboring localities we find the lithological character of those 

 found here. The main, portion rests upon Carboniferous sandstone, 

 while the two disconnected ends of the horseshoe have flow u over Lower 

 Cretaceous rocks. As is usually the case at the points of outflow, the 

 rocks from different altitudes vary considerably in their mineralogical 

 composition. Near the summit of station 36 the rock presents a very 

 handsome appearance. A greenish crystalline paste contains innumer- 

 able white crystals of oligoclase, which are setoff to advantage by ac- 

 companying black crystals of hornblende. Sanidite also occurs. Six 

 hundred feet below the summit the rock is very similar to that described 

 from station 3i ; a white to light-gray paste, inclosing crystals of oligo- 

 clase ; mica occurring very sparingly. To the southwest of station 

 36 is station 37, located upon the above-mentioned red sandstone, but 

 having on either side, north and south, isolated patches of trachytic 

 rocks. Considerable disturbances have taken place at that locality at 

 one time, prior certainly to the eruption of the volcanics, but it seems 

 that their appearance gave rise to new dislocations, and finally a third 

 era occurred, which was again marked by displacements. South of sta- 

 tion 37 a series of sandstone strata that had formerly been forced from 

 their normal position, were still more disturbed by the intrusion of the 

 volcanic mass. Between these strata of sandstone, layers of about 

 equal thickness of trachyte are wedged in, forming apparently a por- 

 tion of the stratified series. (See section.) A secondary dislocation of 

 about 60 feet vertical distance, has brought the layers of trachyte im- 

 mediately opposite those of the sandstone, and the reverse. • 



The La Plata group, at the head of Junction Creek, a tributary of the 

 Animas, is also one of the isolated volcanic regions. Several small 

 patches are adjacent to the mountains, covering either Carboniferous 

 sandstone, or, as was the case at station 42, Lower Cretaceous sandstone. 

 During the following season, 1875, this region will be explored thor- 

 oughly, and more light will be thrown upon the specific characters of 

 the La Plata Mountains. 



With this latter locality the volcanic area of the district surveyed 

 during 1874 is concluded. Small cones, of basalt probably, were visible 

 from a distance in the Cretaceous country, but were not reached in the 

 course of our travels. There remaius now, yet to be considered, the vol- 

 canic area as a whole, and the conclusions that may be drawn from the 

 consideration of the stratigraphical conditions of the rocks. 



It will be observed that at all points near the border of the volcanic 

 area the strata or flows show a surprising regularity in their structure. 

 Scarcely ever does the dip exceed 2° or at most 3°, unless some very ' 

 local disturbances have given occasion to an increase thereof. As we 

 •approach, however, from the outside of this' area toward the interior 

 portions, a marked change in the stratigraphical relations can be ob- 

 served. We find that the high plateaus, the long-continuing bluffs, and 

 the stratoid peaks, all show a tendency to dip off from one point, as 

 from the center of a circle toward its periphery. With this tendency 

 the dip increases gradually, as we approach that region near which sta- 

 tion 10 is located. So far as could be observed, the flows show a dip off 

 from that locality on every side of it, except in the immediate neighbor- 

 hood, where a number of dips are noticed, falling in toward the depres- 

 sion east of station 10. This feature is so constant, is, furthermore, 

 unique throughout the entire district of which this chapter treats, that 

 I could not do otherwise than attach great importance to it. Taking 



