138 
BULLETIN OF THE LABORATORIES 
grains of augite much broken up, always makes up a considerable per 
cent, of the rocks. They also carry quite a large amount of suffused 
iron. 
On the southwest corner of the island, rising above the water’s 
edge several feet, occurs a fine-grained conglomerate with small, sharp- 
ly angular grains. Embedded in this are large boulders from one to 
twelve inches in size, of several different kinds. The question of the 
origin of these boulders becomes of the greatest value in establishing 
the relations of the different series. One of the most numerous is a 
granite, quite close-grained, light colored, containing somewhat altered 
white mica, orthoclase much changed, but still retaining its form, and 
quartz. There are no accessories present. These boulders have all 
been rounded by the action of water. 
No. 1087 (Plate XIII, Fig. 2.) is from a typical mica schist of 
a light gray coloi, dotted all over by minute black crystals, giving it 
somewhat of a salt and pepper color. Microscopically the rock is seen 
to have a cloudy gray magma, in which are scattered very profusely 
small fragments of quartz. Large scales of biotite mica, permeated 
in all directions* by smaller scales of the same, make up the remaining 
ingredients of the rock. Much of the mica is changed to chlorite. 
Comparing this specimen with No. 1051, a mica schist from the high 
cliffs, eight miles west from Dog river, on the main land, we find that 
they are almost identical in appearance and composition. Their color 
in the hand sample is the same as well as all of their macroscopic 
characteristics. Under the microscope we find the same similarities. 
The magma differs slightly, but it is very similar in both. The size 
and relative amount of quartz grains in the two are the same. The 
mica in No. 1051 is more altered and is not aggregated into such .pe- 
culiar masses. Perhaps the most noticeable difference is the presence 
of a few smill crystals of triclinic feldspar in No. 1051, but this can 
be explained by the proximity of the eruptive No. 1050. 
Another interesting example is a boulder of mica diorite which is 
also common in the conglomerate. The plagioclase occurs as very 
numerous small elongated crystals, permeating the section in all direc- 
tions. These have been slightly altered, but can be readily 
identified. The mica has for the most part disappeared, chlorite tak- 
ing its place. This rock can also be readily referred to the igneous 
form, No. 1050, which occurs at the same cliffs, interbedded with the 
schists. As in the preceeding case the chief differences lie in the de- 
