178 Ileadden — Silicic Acid in Mountain Streams. 



It will be observed that the lowest percentage of silicic acid 

 in these residues is 17*9 and the highest 22, while the inter- 

 mediate ones are about 21 per cent. In an analysis made of 

 Foudre water in 1897, I obtained upwards of 36 per cent of 

 silica. My attention was attracted by the general agreement 

 of these analyses, especially by the fact that three of them 

 agree in showing almost the same excess of silica after having 

 combined all of the bases. This is in strong contrast to the 

 residues obtained by the evaporation of ground waters, drain- 

 age waters, or river waters taken in the lower portion of their 

 courses which, especially in the case of the Cache a la Poudre, 

 consist very largely, and at times wholly, of return waters, i. e. 

 such as have been taken from the river, have been used for 

 irrigating purposes, and have found their way back again. 

 The residues from such waters frequently show an excess of 

 bases over the quantity necessary to saturate the inorganic 

 acids. The excess of acids in these analyses is so pronounced 

 that there is no question about its being actually the case. The 

 same is shown by the analyses of the geyser waters and those 

 of the hot springs previously alluded to. There can be no 

 question raised about the silicic acid having been in solution 

 for the waters were perfectly clear, but as a matter of precau- 

 tion were filtered before being evaporated to dryness, and 

 there is no difference in the results whether the evaporation 

 was conducted in copper or porcelain vessels. 



The difference between these waters as mountain streams 

 and when they have become plains streams suggests that the 

 difference observed may be a characteristic of mountain streams, 

 whose waters have not come in contact with any other rocks 

 than granites, gneisses or schists. This suggested that an 

 examination of water from a greater depth might add interest- 

 ing information. I accordingly obtained a sample of water 

 from the Running Lode mine at a depth of 825 feet. The 

 water is clear and of good quality. The vein at this point is 

 tight and there are no ore bodies immediately above this point 

 along which the water has to move. The country rock is that 

 characteristic of Gilpin County, varying from a schist to a com- 

 pact granite. The hanging wall in this instance is frequently 

 composed of a white orthoclase. The vein matter is highly 

 silicified but there is not much kaolin, though there is a little 

 scattered through the ore. The residue from this water gave 

 the following results : 



