112 Scientific Proceedings, Royal Dublin Society. 
include both these products. Further, the destruction of pyroxene 
in weathering frequently involves the conversion of the wollastonite 
molecules into calcite, and this produces great increase in volume ; 
thus :— 
Wollastonite. Calcite. 
CaSi0; + CO, = CaCO; + S10, 
116 10 
Molecular volume, aa = = = = 22. 
The calcium carbonate is almost of the same volume as the wollas- 
tonite, but the total change is an expansion of about 51 per cent. 
It may be of interest to consider the total change of volume of 
a rock consisting of anorthite and augite, when completely altered 
by the weather. For this purpose we will assume a mixture in 
which the silica is equally divided between the two minerals, and 
we will select a pyroxene having the composition found by Teall? 
for this mineral, as it occurs in the Great Whin Sill, while the 
anorthite may be taken as ideally pure. We shall thus have :— 
Pyroxene, . : : . Ca,Meg., Fe ,Al Fe’ Sigs O19. 
Anorthite, | . ; . 11 (CasA1,8i,02.). 
11 Anorthite + 11 Wollastonite? = 11 Zoisite + 11 Quartz 
Mol. vol., 300 x 11 41x11 268 x 11 22°5 x 11 
Change in volume, ‘ : . 8195 — 3751 =— 556. 
8 Wollastonite + 8 CO, = 8 Calcite + 8 Si0,. 
41x 8 40x8 22°5x8 
Change in volume, : : . 500 —- 328 = 172. 
Residual Pyroxene. Pennine. 
Mg,, Fe” ,Al,Fe.Siy,014 = SpAty + 16 810, 
32°38 x 47 = 1518 110x138 225 x 16 = 3860. 
Change in volume, 3 ; 11790 dols)= 2722 
The total change is thus 444 — 556 = — 112, or a contraction of 
112 on a total volume of 5597, i.e. 2 per cent. It is thus evident 
that when‘epidotisation is a prevailing change in the weathering 
of basic rocks the total alteration of volume will not be large, and 
1 Teall, ‘‘ British Petrography,”’ 1888, p. 157. 
* Calcium pyroxene molecules. 
