SUMMARY. 385 



levels, for epidote is far less soluble than chlorite, and under these circum- 

 stances would form in obedience to the general law of precipitation. Its 

 occurrence is usually compatible with this supposition, but it is not so deci- 

 sive as to warrant a positive assertion that the conditions of its formation 

 are those indicated. 



Decomposition of feldspars. — The tricliuic feldspars of the Washoe District 

 retain their optical properties in a recognizable form much longer than the 

 ferro-magnesian silicates. Among the mine rocks it is very rarely that bisili- 

 cates or mica occur undecomposed, but it is the exception when a slide of a 

 tolerably hard I'ock does not show recognizable feldspars. When the feld- 

 spars are altered they are replaced by an aggregate of polarizing grains, 

 which appear to be quartz and calcite with some opaque particles, but with 

 no transparent amorphous material. Kaolin could hardly be present in 

 large quantities without being recognized microscopically. The analyses 

 of the clays, too, show that when allowance is made for the presence of 

 hydrous chlorite tliere is not enough water to corres2Dond to any hvrge 

 percentage of kaolin. In fact the analyses of the clays so exactly cor- 

 respond to the composition of the firm rocks that the great masses of clay 

 evidently represent only equal volumes of disintegrated rock. On the 

 whole, therefore, it appears improbable that tliere has been any great amount 

 of kaolinization in the Washoe District. 



Lateral-secretion theory. — As is Well kuowu, Prof. F. Sandberger has very 

 ably maintained what is known as the lateral-secretion theory of ore depos 

 its. With a view to testing the probabilities of this theory, with reference 

 to the CoMSTOCK, the rocks of the District have been assayed with all pos- 

 sible precaution. The principal rocks containing precious metals were also 

 separated by Thoulet's method, and the precious metals traced to their 

 minei"alogical source. The results of tliis investigation show many inter- 

 esting facts, among which are the following: The diabase shows a note- 

 worthy contents in the precious metals, most of which is found in the augite; 

 the decomposed diabase contains about half as mucli of these metals as the 

 fresh rock; the relative quantities of gold and silver in the fresh and decom- 

 posed diabase cori-espond fairly well with the known composition of the 



CoMSTOCK bullion; and the quantity of precious nielals which has been 

 25 c L 



