OF DENISON UNIVERSITY. 
135 
Fig. 6.) The porphyritic grains are of plagioclase with scattered aug- 
ite. It is thought that much of the granular magma is augitic. The 
ilmenite is altered in many places to titanic iron. 
In discussing the Keweenaw rocks as exposed on Michipicoten 
island, cape Choyye and cape Cargantua, we shall describe only 
enough to give a clear idea of the relations of this series to the older 
Lawrentian and Huronian, as found in this region. We may consider 
these rocks under two heads; first, the eruptive igneous, including all 
the diabases, amygdaloids, pseud-amygdaloids and perhaps the porphy- 
ries ; second, the sedimentary and metamorphic. The former might 
be considered the result of a single series of overflows closely related. 
The cooling of these, taking place under different conditions, as in the 
presence of water, superheated steam, under great pressure or other- 
wise, must of necessity produce many different forms of the same 
overflow. Only those whose structure is in some way characteristic, 
are here described. 
No. 1077 (Plate XIII, Fig. 3.) occurs on the south shore of 
Michipicoten island, forming the most of the islands along the shore. 
It is filled with vein matter, principally of quartz and agate and is 
also somewhat amygdaloidal. The color is black, with almost a metal- 
lic lustre. An uneven fracture and exceedingly tough. The specific 
gravity is 2.76. Under the microscope the rock is seen to be com- 
pletely crystalline. Oligoclase makes up a great portion of the sec- 
tion, occurring in long, fine crystals, generally twinned. Rounded 
grains of augite are scattered irregularly through the mass. Here and 
there a crystal of orthoclase is present, but much altered. A very pe- 
culiar kind of grains, evidently suffused with iron of a bright yellow 
color, without polarization or pleochroism, filled with inclusions, prob- 
ably of gas, with concentric lines running through them, is found. 
There is a very large quantity of magnetite in the rock, not occurring 
in regular form, but rather filling the interstices between the other min- 
erals. According to Rosenbusch this is a typical diabase. 
No. 1082 (Plate X, Fig. i., and Plate XI, Fig. 3.) is a diabase 
porphyrite, black in color, with light colored crystals varying from one 
sixteenth to three-sixteenths of an inch in length, scattered thickly 
through it. The specific gravity is 2.7. The fracture is distinctly 
conchoidal. In the aphanitic magma large twins of labradorite cut 
