2 1 2 The Affuricafi Geologist. October, i9oo 
The birefringence is quite variable according to the degree 
of crystallinity of the mineral; the maximum, however, is at 
least .025. 
All conditions of crystallinity occur in bowlingite, from 
the compact perfectly crystallized state through lamellar and 
fibrous varieties to the earthy noncrystalline condition. The 
last evidence of crystalline character to disappear is the bire- 
fringence, which though feeble can be distinguished when 
all other evidences of crystallization have disappeared. 
The alteration of olivine to bowlingite begins along the 
edges of the fractures so characteristic of the former, and 
spreads inwards. It is thus exogenetic. One cr^^stal may 
alter to a single crystal of bowlingite, or it may alter to a 
fibrous mass of varying orientation, or the two conditions may 
be combined. In the first case the bowlingite is oriented on 
the olivine so that the optic planes are mutually perpendicular 
and n^ of olivine is parallel to Jim of the bowlingite. 
Antigoritc is the mineral of which "serpentine" is or- 
dinarily composed; but in the Minnesota rocks it is much less 
abundant than bowlingite, w^hich seems to be a ferriferous 
mineral of the same group. Antigorite is fibrous or lamellar 
in habit, never compact. It often presents a rudely radial ar- 
rangement of the fibers, and occurs more frequently at some 
distance from the olivine grains than in them. On altering 
the olivine increases in volume and hence its decomposition 
products are forced into the fractures of the surrounding min- 
erals, especially the labradorite. The bowlingite, however, 
never forms at any distance from the olivine ; the microscop- 
ic veins running from one olivine grain to another are always 
composed of antigorite. If an amygdule or cavity of any 
kind occurs it is filled with antigorite; the fractures along 
which the material has been brought. are very distinct; they 
also are filled with antigorite. This is illustrated in plate 
VIII, Fig. I. 
Titanite occurs in the Pigeon point olivine diabase. It 
has apparently formed from magnetite, though the two min- 
erals are now slightly separated. It has a pale yellow color 
without determinable pleochroism. . It occurs in brown bio- 
tite causing an intense pleochroic halo. 
Biotite is formed by mesogenesis between the magnetite 
