TO JUNCTION OF GRAND AND GREEN RIVERS. 61 



the sea. Its surface-rock Avas then Carboniferous limestone, from which the overlying 

 formations had been entirely removed, and in which lines of drainage had been exca- 

 vated, having the general direction and character of the present stream-beds. There 

 is evidence that the cones of the mountains themselves, and the floods of lava which 

 surround them, have been formed by a series of eruptions, and that the latest of these 

 paroxysms occurred within a few years. 



As I have previously remarked, the eruption of the material composing and sur- 

 rounding these volcanoes has produced comparatively little disturbance of the sedi- 

 mentary rocks upon which they rest. We have as yet no evidence that they form part 

 of any well-marked line of upheaval, nor is there any obvious connection between 

 these mountains and any of the chains which surround them, or with any of the other 

 volcanic vents which are scattered over the great central plateau. My attention was 

 particularly directed to this point by Baron Humboldt, through Mr. Mollhausen, before 

 visiting New Mexico ; and I was particularly requested to examine the country be- 

 tween the San Francisco Mountain and San Mateo, to detect, if possible, some connect- 

 ing link, such as lines of upheaval or of volcanic vents. I was, however, able to dis- 

 cover no proof whatever of any relationship between the two other than the perfect 

 correspondence which their local phenomena present. It is true that east of the Little 

 Colorado, just south of the Moqui Villages, is a series of buttes composed of compar- 

 atively recent volcanic matter, which made 'its exit from a number of vents in that 

 vicinity; but the sedimentary rocks are scarcely at all disturbed, even in the intervals 

 between these trap buttes, and the country on either side is entirely free from dikes, 

 faults, or displacements of any kind which would indicate the action of disturbing 

 forces along the line connecting San Francisco Mountain and San Mateo. 



It is a somewhat remarkable fact that volcanic vents similar to those under con- 

 sideration, though of less magnitude, are scattered over the entire area of the central 

 table-lands, from Mexico far up into the British possessions. About many of these the 

 evidence abounds that their fires have only been quite recently extinguished, and yet 

 in none, so far as we know, are they now burning. The cause of this simultaneous 

 cessation of an action, lately so wide-spread and vigorous, becomes an interesting sub- 

 ject of inquiry. 



SAN MATEO (MOUNT TAYLOR). 



This is the second in importance of the great extinct volcanoes of the central por- 

 tion of our continent which have come under my observation. It has an altitude of 

 between 11,000 to 12,000 feet, perhaps a thousand feet less than that of the San Fran- 

 cisco Mountain. It stands quite alone, and has no apparent connection with any other 

 peak or range. Like most isolated mountains, it is composed of erupted material, and 

 seems to have been wholly formed by the acciimulation of ejected matter around a 

 volcanic vent. As has been remarked of the San Francisco Mountain, this volcano has 

 been in vigorous action at a very recent period. The lava-streams of its latest eruptions 

 present precisely the same appearance as those of Vesuvius, when but just cooled; and 

 it is difficult to believe that they have been exposed to the action of the atmosphere 

 even for so much as a hundred years. 



