deere atest al ete ee 
ene — ae 
from the Tilly Foster Iron Mine. 449 
original crystalline composition, and having the external char- 
acters of schiller spar, and other portions, where the change 
is more complete, being devoid of all structure, and true 
serpentine. 
The serpentine of these pseudomorphs has the same dark 
green color as that from the enstatite, and thus differs widely 
from that derived from the alteration of ripidolite. 
8. Pseudomorphs after Biotite, 
The large grayish-black or brownish-black masses of biotite, 
the plates of which are 8 or 4 inches across, are sometimes 
changed to a dark green serpentine. The transition from the 
unaltered part of a plate to the altered sometimes takes place 
through the intermediate stage of a chlorite—the folia becoming 
green and inelastic; then, beyond this, they are purely ser- 
pentine, and lose finally all traces of the micaceous structure. 
As the mineral resembles the chlorite in having cleavage joints, 
though less open, the change has often been limited by divi- 
sional plates; and the altered end of a group of plates looks 
sometimes like a mere juxtaposed mass of serpentine. But the 
structure and surface lining of the biotite may in some parts 
traced into the serpentine. 
9. Pseudomorphs after Dolomite. 
dolomite—the passage of the dolomite grains into the serpen- 
tine being distinct. In other cases imbedded masses of dolo- 
mite have the exterior for a fourth or a half of an inch changed 
to apple-green serpentine, while the interior is unchanged ; the 
cleavage planes of the latter may sometimes be traced a little 
ways into the former, but for the most part the serpentine has 
the ordinary massive or structureless character. Such serpen- 
tine effervesces for a while in dilute hydrochloric acid, owing 
to a portion of dolomite still present. 
10. Pseudomorphs after Brucite. 
One of the imbedded masses of dolomite having an exterior 
of apple-green serpentine—about two by three inches in its 
dimensions—contains fibrous brucite distributed through a 
tags of it, and wholly replacing some of it—as more particu- 
arly described beyond (p. 453)—and this brucite has _partici- 
ted in the change to serpentine undergone by the dolomite. 
‘he fibrous structure of the brucite may here and there be 
traced into the serpentine. 
