1905-1906.] 345 



rock, and the sodium carbonate is supposed to> attack fresh 

 silicates, setting free more silica. 



Both of the above theories, it will be observed, depend 

 upon the decomposition of the mother-rock for the supply of 

 mineral matter deposited in the veins and vesicles, In the first 

 case, the action of the hot alkaline waters, aided by carbonic 

 and hydro-sulphuric acids., is supposed to have been a com- 

 paratively rapid one; while, in the second theory, the decom- 

 position of the rock and deposition of the mineral matter in the 

 cavities by the action of cold, slightly alkaline water, aided by 

 carbonic and humic acids, is supposed to have extended over 

 extremely long periods of time. G. Bischof calculated that it 

 would require no> less a time than 1,296,000 years to< deposit 

 one pound weight of amethystine quartz in, this fashion. 



6. It has been frequently pointed out that trap-rocks and 

 lavas containing nodules and veins of chalcedony are for the 

 most part much decomposed. In some cases they are com- 

 pletely weathered into a saponaceous green earth, from which 

 the chalcedony may be dug out without trouble. Some geolo- 

 gists go so far as to say that the beauty of the chalcedonic con- 

 cretions is in direct proportion to the amount of decomposi- 

 tion suffered by the mother-rock. I have not found this to be 

 the case as a general rule. Agate and chalcedony in the un- 

 altered rock are more difficult to get at, but such specimens are 

 almost invariably much better than pieces from, the weathered 

 rock. Indeed, where the rock is very much decomposed, the 

 chalcedony has also suffered changes, being cracked, stained, 

 and 'devitrified.' This is the case at Carnmoney, for the finest 

 specimens found there are extracted from, the portions of the 

 rock that have suffered the least from weathering influences. 

 Many of the veins in this rock have suffered so« much from 

 percolating surface-waters that the zeolitic or calcitic layer has 

 been entirely removed, and the chalcedony lost much of its 

 native translucency. When such veins are broken open the 

 chalcedony usually drops out because of the clear space be- 

 tween it and the rock-sides, and it is often found to be much 

 weathered. 



