Gabbroid Rocks of Minnesota. — Winchell. 3 5 1 
Penninite in thin section is sensibly uniaxial, usually nega- 
tive; but also sometimes positive. Both varieties occur in 
this rock. The elongation of the common negative penninite 
is positive with extinction sensibly parallel. 
Inclusions are abundant. They consist of magnetite, 
apatite, zircon and rutile. About all except magnetite are 
halos, faint about the rutile needles, even when a mass of 
needles multiplies the effect. (See plate XIX, figure 22.) 
About the apatite the halos are stronger than about rutile, 
but markedly less intense than those about zircon. In all 
these cases the birefringence is increased sensibly; but in the 
case of rutile the increase is so slight as to escape an approxi- 
mate evaluation; about apatite the increase is about .001, while 
about zircon it is at least .002. It can therefore be concluded 
that in a given mineral the increase in the birefringence in 
pleochroic halos is proportional to the intensity of the latter. 
The increase in the refringnce cannot be accurately deter- 
mined, but it apparently follows the same law\ 
Rutile occurs both in the form of acicular crystals noted 
above, and in small shapeless masses from which needles often 
radiate. The needles are strongly striated parallel to the verti- 
cal axis. They present characteristic twinning both "en 
genou" and "en coeur." Rutile is brown with weak pleo- 
chroism : 
//g^ight violet brown. 
wp=yellowish brown. 
In reflected light the masses of needles give a dull light 
green color. The extreme refringence gives a very high re- 
lief. 
Rutile is abundant in the penninite (See plate XII, figure 
2), and shows that one or both of the minerals [pyroxene and 
magnetite*] from whicii the chlorite has been derived were 
quite rich in titanic acid. It is rarely found elsewhere in the 
rock. 
Magnetite perhaps occurred in small quantity as an origi- 
nal constituent of the rock. But the mineral is not rare as an 
alteration product, being especially produced by the alteration 
*Penninite is derived from magnetite by mesogenesis between the 
latter and piagioclasc. But more commonly biotite forms first by mes- 
tjgenesis, and in turn alters to penninite. 
