OF DENISON UNIVERSITY. 
137 
scope the effects of decomposition become very manifest. The large 
crystals of triclinic feldspar have altered to a mineral resembling or- 
thoclase, but very much clouded. The smaller crystals of feldspar 
have likewise changed, though not as much as the larger ones. The 
chief peculiarity of the rock is the alteration of the augite into chlor- 
ite. These grains are, some of them, an eighth of an inch wide and 
so can be tested for their solubility in sulphuric acid. These grains 
are arranged in spherulitic masses, with radiating structure in polyhe- 
dra. There is a large amount of hematite and magnetite present in 
the section. 
No. 1085 is banded sandstone, with alternate layers of brown 
and gray color. The grains are exceedingly minute, situated in a high- 
ly argillaceous magma. They are so small that they are very difficult 
to make out, but some of them are evidently triclinic feldspar from 
their form and angle of extinction. 
No. 1088 (Plate XII, Fig 7.) is a very black rock, with none of its 
ingredients macroscopically visible. It has a shelly fracture and is 
quite brittle. Under the microscope it is seen to have a crystalline 
base of minute needles of plagioclase, in many cases twinned, with 
small grains of augite frequently surrounding them in sort of a pseudo- 
amydaloidal arrangement. Dotting the section are regular crystals, prob- 
ably magnetite. Here and there are found large crystals of plagio- 
clase, with regular form and entirely unaltered. This rock is a typical 
diabase aphanite. It is situated on the southwest corner of the island, 
overlying a bed of conglomerate, which will be described later. 
No. ,1091 (Plate XII, Fig. 4.) is a specimen taken from the zone in 
contact with the conglomerate. It is dark in color, minutely crystal- 
line and crossed in all directions by minute veins. Its specific gravity 
is 2.79. Small crystals of plagioclase are very abundant, also iron 
grains. The vein matter has aggregate polarization. Whatever augite 
is present it is entirely broken up, but still retaining its bright colors 
of polarization and angle of extinction. The amygdules here are 
small, generally greenish in color, without polarization or pleochroism. 
Alteration product much resembling olivine is present in large quanti- 
ties. This rock is very similar to the one figured by Irving, on Plate 
IX, in his “ Copper-bearing Rocks of Lake Superior.” 
The amygdaloids on the northwest corner island have not yet 
been carefully worked up by us. They are highly amygdaloidal con- 
taining large masses of calcite. Plagioclase, together with large 
