TO JUNCTION OF GRAND AND GREEN RIVERS. 75 



the escape of ibises, wliieli :ire generated in enormous quantities. The temperature at 

 the side of the basin is 140; 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 pungent taste. The sulphur is also perceptible by taste and smell; a strong sul- 

 phurous odor, which is distinguishable at some distance, being exhaled from the spring. 

 When cooled, the water has a strong mineral taste, though rather agreeable than other- 

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

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

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

 is of somewhat complex composition, and such as gives it a character unlike that of 

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

 comparison, the water of the I'agosa might be imperfectly imitated by mixing Blue 

 Lick and Congress water and heating the compound to 140. The flow of water 

 from the 1'agosa is very large, and it finds its way into the San Juan by several subter- 

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

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

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

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

 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 jvcres 

 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 I'agosa; and it is evident its waters issue 

 from the bitumino-calcareous shales forming its middle division. These shales are of 

 marine origin, have a thickness of over 1,000 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 off 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 I'agosa, the hot water is doubtless derived from a great 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 wliich really exist in this portion of the continent; or, more generally, to 



