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 1 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 1 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. 
