458 J. D. Dana on Serpentine Pseudomorphs, etc. 
6. The era occupied by the serpentine-making and the accom- 
panying changes may have been long; and yet there are no 
facts that definitely prove this. The great extent of the change 
suggests that it was long. But the successive depositions of 
serpentine and other such changes may have been effecte 
to farther progress so that the block was finished out com- 
plete before proceeding to the next: as in the cubic pseudo- 
morphs (p. 375), chlorite (p. 881), biotite, and the mineral de- 
scribed on p. 450. Hence side by side positions of altered and 
wholly unaltered portions are common in such minerals. This 
law must be a general one for pseudomorphic changes. 
tine and carbonate of lime which go under the name of Hozoon, 
that is, if they are not of organic origin. 
8. The facts at the Tilly Foster iron-mine afford an example 
of the production of serpentine, not by the help of the mag- 
nesian waters of the ocean or springs, but through the altera- 
tion of magnesian minerals. It is true that the serpentine of 
the fissures is not all of it properly pseudomorphous, since part 
is a deposit at a distance from the crystal or crystalline mass 
that afforded it. But it is all a result of the alteration of mag- 
nesian minerals. 
2. Changes during the epoch of Metamorphism. 
_ ripidolite, magnetite and dolomite, belongs (1) to the ee 
Archean rock cluded, 
