114 Scientific Proceedings, Royal Dublin Society. 
The total change is now a gain of 2140 volumes on 5852, equiva- 
lent to 36°4 per cent. 
Since anorthite will rarely be converted into epidote alone, but 
will usually give rise also to kaolin, it is probable that, on the 
whole, the bulk of the material resulting from the weathering of 
a basic rock will considerably exceed that which it possesses in the 
unaltered state. 
No one can have examined the slates and other rocks of the 
Cambrian and Ordovician systems in Leinster and elsewhere, with- 
out being impressed with the frequency of the association of chlo- 
rite with quartz in the infiltrated veins so common in these rocks. 
This finds an easy explanation in the simultaneous liberation of 
chlorite and: silica during the weathering of rocks contaiimg 
pyroxene; and such rocks are of the commonest occurrence in 
the districts traversed by the chloritic quartz veins. 
Before concluding the long digression which our observations 
on the fissured condition of the epidotised rock have suggested, it 
may be worth while to inquire into the volume-change accom- 
panying the conversion of orthoclase into muscovite and kaolin. 
Muscovite may be supposed to arise by the following reaction :— 
Orthoclase. Muscovite. 
3 Al, K.81,0i, = Al, K,H,S1,004 + K,CO; ar 12 Si0,. 
Mol. vol., 3 x 218 274 256202225 sas 
The change in volume is 668 — 658 = 10. 
Thus, on the whole, there is practically no change in volume ; 
but, if the muscovite be considered alone, there is a condensation 
of 658 to 274 volumes. Unless, therefore, some of the free silica 
be deposited as quartz along with the muscovite, we should expect 
to find the development of the last-named mineral accompanied 
by marked signs of shrinkage in the orthoclase. 
The conversion into kaolin is as follows :— 
Orthoclase. Kaolin. 
Al, K.$1,04. = H,A1,8i,0, oP K,CO, + 4 $10,. 
Mol. vol., 218 100 62 90. 
The change in volume is 252 — 218 = 84. 
The expansion, on the whole, is slightly larger than in the 
