Headden — Silicic Acid in Mountain Streams. 173 



Kio Grande del IN'orte in the same measure as to the Poudre 

 and other streams mentioned in this paper. In these cases we 

 cannot assign the presence of the silicic acid in the waters to 

 the causes usually cited as explaining it, and I do not believe 

 that it is necessary that we should even in other cases. 



In regard to the San Luis Yalley it should be explained that 

 there is a bed of volcanic ash, bombs and even a sheet of lava 

 at or near Antonito, of no great northward extension, this 

 place being apparently at its northern limit, and it is doubtful 

 whether the silicic acid in the artesian well waters is derived 

 from felspars of volcanic origin, either in the form of ash or 

 lava, as the wells are driven through the clays, shales, and sands 

 deposited in the bed of the lake formerly tilling the valley. If 

 any doubt were entertained of the origin of these strata, the 

 fish vertebrae, shells and fragments of wood found in them 

 would dispel it. The most striking character of the sands is 

 the presence of kidney-shaped concretions of calcic carbonate, 

 having a smooth exterior coating. Some of them are filled 

 with a cellular mass, others are composed of concentric layers 

 of calcic carbonate which, like the exterior one when scaled off 

 from the interior mass, is, owing to its extreme thinness and 

 smoothnesS; somewhat translucent. The other grains making 

 up these sands are fragments of granite, some of them even 

 showing particles of pyrites, quartz both light and dark, and 

 grains of felspar. There may be some grains of eruptive rocks, 

 but I am not fully enough convinced of this to assert their 

 presence. 



The granites of the respective areas are the metamorphic 

 granites of this region, many of them closely resembling the 

 Pike's Peak type, others identical with them. 



The residue obtained by evaporating the water of the Kio 

 Grande del ]^orte, sample taken near the town of Del E"orte, 

 contained 27'15 percent of silicic acid; that from an artesian 

 well in the town of Alamosa, Spriesterbach's Well, 27*0 per 

 cent ; from a large spring in the southern or rather southeastern 

 portion of the valley, Mcln tyre's Spring, 29-64: per cent ; from 

 the McNieland Well 36 per cent ; and that from the Bucher 

 Well 46-9 per cent of silicic acid. 



The conditions under which these waters occur preclude the 

 application of the theory advanced to explain the presence of 

 silicic acid in the waters of geysers and hot springs in general. 

 The maximum temperature observed in any of the springs or 

 wells in the San Luis Yalley was 71° F. and the minimum 53° F. 

 These waters do not issue from, and probably do not, in any 

 portion of their course, pass through eruptive rocks of any sort, 

 except it be as surface waters in the lava cappings prevalent to 

 the west of this valley. The river and artesian waters probably 



