172 Headden — Silicic Acid in Mountain Streams. 



applicable to the river waters of our mountains and it is very 

 doubtful whether they would apply at all to any of the springs 

 occurring in the San Luis Yalley. It is true that eruptive 

 rocks occur in the southern part of the valley ; it is also true 

 that some of the springs mentioned later are located on the 

 northern edge of this belt of lava occurrences. I do not, how- 

 ever, believe that the presence of silicic acid in the waters is 

 influenced by the lava which occurs here as remnants of a 

 sheet which formerly covered the country, especially to the 

 southward, but whose point of eruption was probably quite 

 remote. If these waters do come in contact with eruptives of 

 any sort in their course to the surface, they are cold and the 

 waters would act upon them as they would if they were on the 

 surface plus whatever effect the pressure, molecular or other, 

 under which the water exists in the rocks may produce. These 

 springs are bringing no subterranean heat to the surface, the 

 temperatures observed being 59° F. and 70° F. There are 

 warm and hot springs south and southwest of this, but they are 

 many miles away and the water is relatively poor in silicic 

 acid ; that of Ojo Caliente, temperature 122° F. contains about 

 75 grains total solids per imperial gallon, of which less than 

 one per cent is silicic acid. The range of the temperature of 

 the artesian wells throughout the valley is from 53° F. to 71° 

 F. It seems then that the part played by heat in taking the 

 silicic acid into solution in these cases is as good as nothing, 

 while in the case of the rivers receiving their supply directly 

 from fields of melting snow it is patently out of the question. 

 All the statements that I recall relative to the source of silicic 

 acid occurring in waters derive it principally from the felspars, 

 to which I think no serious objection can properly be taken. 

 The contexts as well as the examples given lead one to infer 

 that it is derived from the felspars of eruptive rocks. Such a 

 supposition would not be admissible in this instance. The 

 waters, especially of the rivers, are surface waters, the volume 

 of which in the summer season varies regularly with the periods 

 of sunshine and darkness, twelve hours of greater and twelve 

 hours of lesser flow, corresponding to day and night. Their 

 flow in autumn and winter is greatly diminished owing to the 

 retention of the surface waters in the form of snow and ice. 

 There are no large springs nor any considerable number of 

 small ones except such as are supplied by the soil-covered val- 

 leys of the mountains and whose waters are likewise surface 

 waters. There is comparatively a very small amount of igne- 

 ous rock occurring in their various drainage areas. 



In the case of the Cache a la Poudre there are a few por- 

 phyry dikes and one inconsiderable body of lava within its 

 drainage area. These observations are not applicable to the 



