198 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



ward from the north side, producing the dip by virtue of any such flow. 

 On the contrary, it has come from the west. Traveling westward to 

 the head waters of the small creek (we were camped upon a tributary of 

 Godwin's Creek), it will be seen that with increasing elevation the ba- 

 salt ceases, and a number of small hills are formed by the rhyolite. In 

 any other direction than westward, only small isolated patches of rhyo- 

 lite are found, fewer of them toward the east. Considering all these cir- 

 cumstances, the conclusion presents itself that we here have an instance 

 of intrusive basalt, having become intrusive alter the flow or flows of 

 rhyolite had already assumed a state of rigidity. In no case within the 

 district besides this one have 1 found these two rocks in the same rela- 

 tive position. In speaking of the country drained by Lake Fork Creek 

 and its tributaries, I shall have occasion to enter into the detail of the 

 mineralogical character of this and several other interesting rocks 

 found at that locality. 



Dolerite and basalt both occur in this district, the latter in by far 

 greater quantities, however, and in a number of varieties. The three 

 plateaus upon which the stations 3, and 4, 5, and 19 and 20 are located, 

 are capped by a heavy layer of basalt. Generally the color is a dark 

 gray or black, but in a number of instances it was found to be a brownish- 

 red, produced by decomposition of the magnetite it contains. These 

 three plateaus strike in one line, approximately north and south, and a 

 former connection between the two more northerly ones seems highly 

 probable. Station 3 has an elevation of 12,669 feet, while station 5, 

 seven miles distant in a straight line, is only 101 feet higher. Both are 

 located on the summit of the plateaus, and their elevation shows that 

 there can have been but very little change in the niveau of these beds 

 independent of each other. It has been mentioned above, that the 

 highest members of the trachytic group are to be found in the western 

 part of the district. This is probably owing to the fact that the eastern 

 portions were, ao the time of eruption, too high to be reached, a view 

 sustained by the observation just quoted. We find the basalt of these 

 plateaus resting upon trachyte No. 2, except at stations 19 and 20, 

 where it covers No. 3. On the summit of the Rio Grande Pyramid 

 (station 21), we find the same case occurring, a cap of basalt, 600 feet in 

 thickness, forming the highest portion of the peak. Isolated patches of 

 basalt occur at a number of points, forming either the caps of peaks, or 

 presenting, in the lower southerly country, single eruptions of but 

 small extent. Station 18, on the north side of the Rio Grande, east of 

 Pole Creek, shows a cap of this kind. East of station 10, in the narrow 

 canon, basalt also crops out, underlying the rhyolite. In its specific 

 features it is quite interesting at that locality, and shall be spoken of 

 more at length hereafter. In color it is there almost black, weathering 

 brown on the surface. Small particles of olivine are contained in the 

 microcrystalliue paste. Prisms of augite occur sparingly. Altogether 

 the rock has a more crystalline appearance than basalt generally shows. 

 "With basalt the list of volcanic rocks found in our district is exhausted, 

 and we shall proceed to give a synopsis of their horizontal distribution 

 according to the various drainage systems. 



A line drawn northward from Del Norte on the Rio Grande, to Sa- 

 guache, would approximately give the eastern border of the volcanic 

 rocks. It is there that the bluffy character is well developed, continu- 

 ing westward for some distance up the Rio Grande. Mostly of a brown 

 to reddish-brown color, the perpendicular walls, sometimes several hun- 

 dred feet in height, give to the country a very characteristic aspect. 

 Trachyte No. 2 composes these bluffs, allowing an exposure of No. 1 at 



