454 J. D. Dana on Magnetite Pseudomorphs 
he suggests, from mixture with a little serpentine. The rest 
was soluble in boiling acid, though with more difficulty than 
ordinary brucite. 
The above observations lead to the following conclusions. 
(1) That the brucite in the veins of the ore-bed was made out 
of dolomite, one of the constituents of the ore-bed; but (2) 
that its production took place at the time of the erystallization 
of the dolomite of the same veins, and while chondrodite was 
in process of crystallization, and probably during the period 
of metamorphism. Hence the case is not one of true pseudo- 
morphism, that is, of alteration of crystals of dolomite to bru- 
cite, yet still the brucite was formed out of the dolomite of the 
enclosing rock. 
r the brucite associated with serpentine over the 
blocks of the chondroditic rock of the ore-bed, the origin was 
of later date, the epoch of the serpentine changes, but proba- 
bly the same in kind. 
The change of dolomite to brucite, under the action of heat 
and moisture, or heated mineral solutions, loses part of its 
extraordinary aspect, when it is remembered that, by Pattin- 
son’s process for obtaining magnesia, the subjection of dolo- 
mite to heat separates the carbonic acid from the magnesia portion, 
leaving the carbonate of lime intact, and then the magnesia 
is removed by carbonated waters. 
- C. Maeyetire Psevpomorpas. 
13. Magnetite Pseudomorphs after Dolomite. 
Dolomite often occurs in the cavities or veins of the ore-bed 
in groups of rhombohedral crystals, which are often of large 
change evidently began in each case at the surface, but not in 
all cases over all the surface planes at once; and sometimes it 
extended down into a crystal along rifts. The magnetite of a 
rhombohedral face shows its luster, as the crystal is turned to 
the light, successively in one large patch after another; thus 
indicating that the deposition sth at different points and 
spread laterally; the group formed from each such center hav- 
