Gabbroid Rocks of Mmnesota. — WincJiell. 183 
The diabase is of a mottled gray color often slightly green- 
ish; the ophitic texture passes occasionally into a porphyritic- 
ophitic texture by the appearance of labradorite phenocrysts, 
whose polysynthetic twinning on cleavage surfaces is quite 
conspicuous. Dark green to black augite is abundant and 
magnetite never fails, though it is sometimes only sparsely 
disseminated. Olivine is rarely entirely wanting. 
On decomposition the rock usually becomes darker, as 
fibrous and compact hornblende appear and biotite forms in 
bright lamellae. 
Under the microscope the ophitic texture is at once seen, 
the labradorite usually occurring in lath shaped forms, having 
crystallized before the pyroxene. Nevertheless the grain is 
so coarse and the labradorite so abundant that such rocks have 
repeatedly been called gabbros by writers on the petrographv 
of northeastern Minnesota. The order of crystallization is: 
apatite, magnetite, labradorite, pyroxene. If any olivine oc- 
curs, its period of crystallization is in general later than that 
of the labradorite, as is shown by the numerous crystals of 
feldspar projecting into the olivine, but exceptions to this rule 
are not lacking. Pyrite can occasionally be found in ver\ 
small amount, which crystallized about the same time as the 
magnetite, being often enclosed within the augite and labra- 
dorite. 
The ?mncrals of the normal diabase differ in scarcely any 
respect from those of the olivine diabase which will be de- 
scribed in detail in the next chapter. The labradorite varies 
in composition between Aba An4 and Abs An^. The pyroxene 
has a very slight pleochroism in the greenish and pink tints, 
very similar to the colors of the abnormal pyroxene of the 
olivine diabase from Pigeon point; but the pyroxene of the 
typical diabases seems to be normal augite. Olivine is always 
in much less quantity, and may even lack entirely in these 
types. Magnetite, apatite, and pyrite present no special char- 
acters. The commonest minerals of secondary origin are: 
biotite, hornblende, penninite, muscovite and calcite. 
Inchisions — The labradorite frequently contains minute 
black dust-like inclusions as well as opaque black needles. 
■'Microplakites" and "microphyllites" have not been observed, 
nor has any iridescence appeared. Liquid inclusions occur 
rarelv. 
