368 The Americati Geologist. Decembtr, i9(;»» 
iiishes good evidence as to the origin of the rock. It is here 
reported for the first time in a granular basic rock of the labra- 
doritic series. Its twinning is characteristic when all other 
methods of distinction fail. Cordierite contains many inclu- 
sions of magnetite, often coated with spinel, of liquids, and 
finally of staurolite. The last is oriented definitely in its host- 
But no pleochroic halos occur. 
Muscovite only occurs as a product of alteration, and is 
very rare on account of the fresh condition of the rocks. 
Biotitc occurs as a primary mineral in only one type, the 
cordierite noryte, in which the optic properties prove the ex- 
istence of the unusual variety, miouiitc. In some of the gab- 
bros and diabases biotite is not uncommon, but it seems to be 
invariably of secondary origin (often mesogenetic) in these 
cases. Pleochroic halos in l)iotite are an interesting subject 
of study. 
Bro?izite and enstatite occur in only one rock, the cordier- 
ite noryte. Their presence is due to a lack of calcium in the 
rock.* Bronzite greatly predominates. They illustrate a new 
type of uralitization in their alteration to anthophyllite. 
Diopside {Pigeonite) is not known to occur except in the 
olivine diabase from Pigeon point, though a similar mineral 
exists in the orthoclase gabbro. In the former rock it pre- 
sents most remarkable anomalies, consisting essentially of a 
very small, but variable value of the angle of the optic axes, 
though the pyroxene presents no peculiarities in its chemical 
composition except an unusual richness in titanium. 
Aiigitc, with its modified form diallage, occurs not infre- 
quently in the Minnesota diabases and gabbros, and in very 
small quantity in the plagioclasytes, troctolytes, and silicofer- 
rolytes. In the modified types, such as the orthoclase and 
quartz gabbros, the augite is much altered, producing horn- 
blendcj biotite, chlorite, etc. Augite, as well as diallage, often 
shows brownish lamellar inclusions, considered to be of pri- 
mary origin. 
*The remarkable rock (from near Harrismith. Orange Free State) 
described by M. Molengraaf (Jahrb. k. preuss. geol. Landesanst. Ber- 
lin, 1877, p. 44) apparently presents an exception in both these par- 
ticulars. It is essentially composed of a glassy groundmass rich in 
crystals of cordierite and spinel with small crystals of augite (?) and 
magnetite. 
