THE LESTER RIVER BEDS. 281 
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the section. The amygdules are thickly crowded, small, often elongated, 
and chiefly composed of radiating laumontite in the specimen brought away. 
The fine-grained, conchoidal-fracturing, brownish rock, with accompanying 
laumontitic amygdaloid, seen on Encampment River, in the.S. E. 4 of See. 
10, T. 53, R. 10 W., is probably to be placed with the fine-grained rocks 
of the Lester River Group. 
It was not possible to determine whether all of the coarse-grained 
rocks of the Lester River Group are interbedded flows and not dikes, but 
most of them are plainly the former, and the rocks of those exposures 
whose relations were doubtful are in all respects identical with those of the 
undoubted beds. As an example may be mentioned the rock quarried 
below the mouth of Chester Creek. The thin section of this rock is figured 
at Figs. 3 and 4 of Plate II, and is further described in the table on page 
46. It is a medium-grained, highly crystalline, black, rough-textured, 
olivine-gabbro or diabase, consisting chiefly of anorthite and diallagic augite, 
and containing also large particles of olivine and titaniferous magnetite. 
Externally it presents a luster-mottling, such as is seen in the finer rocks, 
and from the same cause. It is the same rock that forms the few coarse 
beds of the Duluth Group, the uppermost amygdaloids of which group it 
closely overlies. It forms a bed of very considerable thickness, and can 
be followed along the lake shore for many rods, varying somewhat in 
coarseness of grain. A similar rock closely overlies the fine-grained dia- 
base of the mouth of Lester River. In it the olivines are more highly 
altered, being almost wholly changed to a brownish ferrugimous substance. 
This layer can also be traced for a long distance on the shore. Similar 
rocks show again on French River, near the north line of Sec. 7, T. 51, R. 
12 W., and in a great ledge 200 feet high on the west line of the S. W. 4 
of Sec. 26, T. 53, R.11 W. The rock on French River occurs plainly in- 
terbedded with the fine-grained rocks already described. It is moderately 
coarse in grain, gray, minutely spotted with red, and of a rough texture. It 
consists chiefly of anorthite, diallagic, very fresh augite in large crystals, 
each one of which includes several detached areas, and olivine, which 
occurs in large patches crossed by black, brown, and red bands of iron- 
oxide. The thin section is represented in Figs. 1, 2, and 4 of Plate III. 
