TO JUNCTION OF GRAND AND GREEN RIVERS. 75 



the escape oi gases, which arc generated in enormous quantities. The temperature at 

 the side of the basin is 14()' ! ; in the center, perhaps something higher. The gases 

 with which the water is charged consist, in part at least, of carbonic acid and sulphu- 

 retted hydrogen ; the former giving it an effervescence, like that of soda-water, and the 

 same pungenl taste. 'The sulphur is also perceptible by taste and smell: a Btrong buI- 

 phurousodor, which is distinguishable at some distance, being exhaled from the spring, 

 \\ hen cooled, the water has a Btrong 1 mineral taste, though rather agreeable than other- 

 wise. It was freely drank by all our party while in this vicinity with no unpleasant 

 effects, hut, on the contrary, a decided sharpening of the appetite. No analysis has 

 yet been made of it, hut it is evident that the large amount of saline matter it contains 



is ol somewhat complex composition, and stich as oives it a character unlike thai of 

 the water of any other mineral spring with which I am familiar. To make a rude 

 comparison, the water oi' the Pagosa might he imperfectly imitated by mixing Blue 

 lack and Congress water'and heating the compound to 140°. The flow of water 

 from the Pagosa is very large, and it finds its way into the San Juan by several Bubter- 

 terranean channels. The deposit from it is very copious, generally snow-white, 

 hut in many places bright pink or green in color. It floats in crusts which revolve 

 upon the surface of the basin 1 have described, and which envelop the bodies of 

 water-beetles, frogs and snakes that have incautiously plunged into it. When first 

 precipitated this deposit consists, in a great degree, of chloride of sodium, but mingled 

 with this are silicates — probably of soda, lime, and potassa — which form a hard and 

 indestructible white rock. In the lapse of ages an immense amount, of this material 

 has been thrown out from this spring. All the low, broad mound, which I have men- 

 tioned as surrounding the basin, is composed of it, and over an area of ten to twenty acres 

 it has a, thickness of at least twenty feet. This is shown on the banks of the San Juan, 

 which is for some distance bordered by walls of this material, having an altitude of 



twelve to fifteen feet 



The geology of all this region is Cretaceous. The upper members of the series 



are exposed in all the hills surrounding the Pagosa; and it is evident its waters issue 

 from the bit iintino-calcarcoiis shales forming its middle division. These shales are of 

 marine origin, have a thickness of over 1, ()()() feet, contain many animal remains, and 

 are highly charged with salts and sulphur; in all these respects resembling the Hamil- 

 ton group of the New York geologists. I think it is true that mineral springs more 

 frequently flow from rocks of this character than any other, and I have been led to 

 suppose that the water of such springs derives its peculiar character immediately from 

 the rock from which it emanates. It is certain that hot water, forced through these 

 strata, would dissolve and carry oil' many of their constituent elements, and would 

 become the agent by which an extensive; range of chemical combinations would take 

 place. In the case of the Pagosa, the hot water is doubtless derived from agreat depth, 

 yet it is entirely possible that the gases and salts which it contains are all derived from 

 the superficial strata through which it passes. 



THE SIERRA SAN JUAN. 



This name has been applied by geographers to several distinct mountain 

 ranges which really exist in this portion of the continent; or, more generally, to 



