from the Tilly Foster Iron Mine. 379 
from the dolomite; or through a solution of a magnesia-silicate, 
that is, of serpentine itself? The former supposition can no 
etrue. For if a block of dolomite had been changed to ser- 
pentine in that way, it would have been changed also in dens- 
ity, and therefore in size; and the difference in size would have 
been apparent by displacements throughout the pseudomorph- 
ous mass. The fact is, all parts are fitted together as exactly 
as if the whole were of one mineral. It follows then, if the 
_ serpentine has displaced dolomite (instead of the original cubic 
mineral), that the material introduced in solution to effect the 
change was not silica, but a magnesia-silicate. It follows also 
_that the same magnesia waters had the power of dissolving, and 
so removing the dolomite; and that the infiltering magnesia- 
silicate took the place of the dolomite as the removal went on; 
it thus being a case of substitution and not of alteration. 
If the first of the above suppositions is true, there was no 
change of dolomite to serpentine, but only of the original cubic 
mineral to each dolomite and serpentine—part to one, and then 
the rest to the other; and both must have been cases of substi- 
tution or removal, in order that the blocks should fit together 
with the exactness characterizing the pseudomorphs. 
2. Hexagonal prisms, probably after Calcite. 
have a thick coating of serpentine, which externally is 
smoothly rounded and shining. In figure !, two of the prisms 
of a group are shown partly denuded of the coating, while a 
third has still the coat on. This coat is a tenth to a sixth of 
an inch through, ahd is transversely semi-columnar in structure. 
Beneath this coating there is usually some incrusting white 
dolomite, granular-crystalline in structure; and the base of 
form; but so large and long prisms of dolomite trihedrally 
w in the pseudomorphs is an aggregation of crystalline 
grains instead of having the cleavage of a simple crystal, it is 
not part of the original mine 
