108 Scientific Proceedings, Royal Dublin Society. 
By subsequent hydration, and exchange of material, the anor- 
thite has been converted into epidote and zoisite, and the olivine 
into chlorite; calcite also has been produced probably by the 
weathering of the anorthite and of the wollastonite molecules in the 
augite. In connexion with these subsequent changes stand the 
development of cracks in the highly epidotised portions of the rock 
and the migration of chlorite from its place of origin to surround- 
ing minerals. No doubt the cracks associated with epidotisation 
are best developed in those portions of the rock which were origi- 
nally in a glassy state, and which might consequently have been 
expected to undergo considerable contraction in the course of devi- 
trification ; but at the same time it is clear that where they are most 
strikingly displayed, the cracks have been produced after the for- 
mation of epidote, since single cracks may be traced passing con- 
tinuously through several crystals of this mineral. A slice in which 
this is conspicuously shown, consisting chiefly of epidote, resembles 
a diagrammatic section of a country traversed by mineral veins. 
The fissures, some of which completely cross the slice, vary in 
breadth in different parts of their course, branch, and reunite, 
forming “horses,” receive feeders and give off droppers, die out 
and are replaced by fresh ones running in the same direction. 
They are now filled with crystalline calcite. It might be conjec- 
tured that under the earth-pressure which has produced “ auswei- 
chungs-clivage’’ in the spilite, the hard and resistant epidote 
became fractured in the direction of the pressure; but even so the 
cracks are so wide and numerous as to suggest a previously exist- 
ing want of compactness; and this one would naturally expect to 
result from the shrinking which it would seem must necessarily 
occur when anorthite gives rise to epidote. Ii, for simplicity, 
zoisite, instead of epidote, be compared with anorthite as regards 
its molecular volume, we obtain the following :— 
Anorthite. Wollastonite.! Zoisite. : 
3(CaA1,8i,0¢) + 8i0,Ca + H,O = H,Ca,A1,Sig02, ar 810, 
Molecular weight,” 810 116 884 60 
1 By wollastonite is to be understood the calcium silicate molecule of pyroxene. 
2 The atomic weight of aluminium is taken as 27 in this and subsequent calcu- 
lations. 
