415 



According to Mr. BECKER (1) it extends for a distance of about four 

 miles parallel to the direction of the lode, and about two miles at 



rictfit angles to this direction. The lode itself lies on the western side 



f 



of the decomposed area. The dominant rocks in this area are horn- 

 blende- and pyroxene-andesites and their holo-crystalline representatives. 

 The alteration has affected more especially the ferro-magnesian con- 

 stituents augite, hornblende and mica. Chlorite has been extensively 

 developed, and epidote has frequently been subsequently formed at 

 the expense of the chlorite. Pyrite also occurs largely in the altered 

 rocks. The felspars have been less affected than the ferro-magnesian 

 constituents, but in extreme cases they also have lost their indi- 

 viduality. The widely - disseminated chlorite has often extended 

 beyond the boundaries of the original ferro-magnesian constituents 

 and penetrated into cracks and cavities in the more or less altered 

 felspar. As a result of this the altered rocks have a greenstone-like 

 aspect It is these altered rocks which constitute the " propylite " 

 of VON RICHTOFEN and ZIRKEL. Mr. BECKER has shown that in the 

 Washoe District propylite has no claim to be regarded as' an inde- 

 pendent rock. The name is one Avhich has been applied to altered 

 forms of different rocks. The chemical re-agents required to produce 

 the changes which have taken place in the Washoe rocks are 

 water, carbonic acid and sulphuretted hydrogen. A stream of water 

 met with at a depth of over three thousand feet was found to be 

 heavily charged with sulphuretted hydrogen and to possess a tem- 

 perature of about 77. Carbonic acid is also known to exist in the 

 waters of the mine. The development of pyrites in the rocks is 

 thus accounted for by the action of sulphuretted hydrogen. 



An important question arises as to the connection between the 

 lode itself and the area of decomposed igneous rock. Have the 

 minerals or ores of the lode been derived from the surrounding 

 rock? Mr. BECKER states that the unaltered rock (diabase) contains 

 a noteworthy amount of the precious metals, which are found to occur 

 mostly in the augite. The altered rock on the other hand contains 

 only about half as much as the fresh rock. It is worthy of note 

 that in the district of Schemnitz, in Hungary, we have also an 

 association of mineral veins with the so-called propylite. 



We have now to refer to cases of the deep-seated alteration 

 which may reasonably be regarded as the effect of more powerful 

 agencies than those already considered, and especially to a kind of 

 plutonic fumarole- or solfataric action in which compounds of fluorine 

 and boron play an important part. That such compounds are potent 

 agents in decomposing granitic rocks and developing new minerals is 

 rendered highly probable by many facts relating to the association of 

 minerals in Cornwall and Devon ; amongst which we may mention the 

 conversion of granite in the neighbourhood of joint planes (see ante, 



(I) Geology of Comstock Lode, Washington, 1882. p. 72, 



