Sottas—On the Variolite and Associated Igneous Rocks. 113 
slight difference in the proportions of the minerals constituting 
the rock may determine whether, on the whole, an expansion or a 
contraction shall take place. The presence of a small quantity of 
olivine in the mixture we have assumed would, of course, tend 
towards expansion. If only as much olivine be added as contains 
6 molecules of silica, of which already 182 molecules are present in 
the mixture, the contraction will become a slight expansion, thus: 
Pyroxene. Olivine. Pennine. 
Me,,Fe’sAlFel”SigO + Mg,Si60, = SpiAt, + 14 Si0, 
1518 42°5 x 6 110x177 22°5x 14 
Change in volume, . . . 2185-1779 = 412. 
In the previous case when no olivine was present the expan- 
sion for pyroxene was found to be 272. This deducted from 412, 
the expansion now found, leaves 140, by which we must increase 
the previous result, which we found to be a total contraction of 
112, thus: 140 — 112 = 28, so that on the total change we have 
now a gain of 28 or 0-4 per cent. 
The conversion of anorthite into kaolin and calcium carbonate 
would bring about a very marked expansion ; thus :— 
Anorthite. Kaolin. 
CazA].S1g024 aP 38CO, ap 6H,O = 3H,A1,$1,0, + 38CaCO3. 
Mol. vol., 300. 300. DOs 
Thus while the epidotisation of anorthite is accompanied by a con- 
traction of about 14 per cent.; its kaolinisation, on the contrary, is 
accompanied by an expansion of 40 per cent.! If, in the mixture 
we have just considered, we suppose kaolinisation instead of epido- 
tisation to occur, we obtain the following :— 
11 Anorthite = 383 Kaolin + 83 Calcite. 
- 3300 3300 1320. 
Change of volume, : ; - 4620 - 3300 = 1820. 
19 Wollastonite = 19 Calcite + 19Si0,. 
10D 760 427. 
Change of volume, ‘ E . 1187 — 779 = 408. 
47 Pyroxene + 6 Olivine = 17 Pennine + 14 SiO). 
1518 255 | 1870 315. 
Change of volume, ; HL 2186) 17738 = 412. 
1 Possibly in this will be found an explanation of the variety of complete kaolinisa- 
tion of anorthite. 
SCIEN. PROC. R.D.S., VOL. VIII., PART I I 
