ON THE NORTHWEST SHORE OF LAKE SUPERIOR. 389 
to the action of gas permeating the rocks from below, but I am inclined to attribute 
it to the action of water. The same phenomenon occurs at many other places on 
the lake-shore, but it has not been observed at any point inland where similar rocks 
occur in situations removed from the constant action of water. About the centre 
of the chain, which encloses the bay on the lake-side, the rocks are polished and 
excavated, to the depth of four or five feet and the width of ten or twelve. This 
great gutter is literally covered with small parallel grooves and scratches, and 
extends the whole length of the exposure. The rock is an exceedingly inde- 
structible one, and there is not a groove or scratch in the direction which masses 
of ice, moved by the present waters of the Lake, would be compelled to take. 
The entrance to the bay is through a narrow gap in the rock just described, with 
barely water sufficient to permit a Mackinay boat to pass. The bay, which is 
sheltered from every wind, is situated between two ridges, about six hundred yards 
apart, bearing east and west; and its peculiar shape is given it by a north-30°-east 
dike, which shows itself at the extreme eastern end of the first ridge, and forms the 
barrier between Bitobigungk and the great bay below, and two other dikes with the 
same bearing, which are seen low down in the next bay. The trough between the 
east and west ridges, as seen at other points, contains a thick deposit of soft shales, 
and in them the basin of the bay has been excavated. 
In describing the rocks on the lake-shore between Bitobigungk Bay and Wisa- 
codé River, I shall begin at the last-named place and proceed westerly. 
In the bay immediately above the mouth of the Wisacodé, are several low expo- 
sures of metamorphosed clay-slates (No. 73), associated with a basaltic bed (No. 
72). The joints of the altered slates present a remarkable appearance, having 
been filled, apparently, with melted matter of the same rock. No. 73 continues 
along the shore, past the small rock-islands, as far as the deep bay into which 
Diarrhoea River empties. It is in low exposures, at the points separating the bays, 
and occasionally projects from beneath the sands of the beach. At one locality, it 
is overlaid by a bed of trap. In the bottom of the bay there is a greenstone dike 
(No. 74), twenty-six feet wide, and bearing east 10° north. At the junction of the 
dike with the sedimentary rock, large cells are developed in the latter (No. 75), 
which contain laumonite and other zeolites. The joints of the trap are encrusted 
with heulandite. The first range of hills along this part of the coast, are from two 
and a half to three miles back from the shore, and bear northeast and southwest. 
Near the west end of the bay the altered slate dips southeast 14°. At the 
point it is overlaid by a bed of trap. At the point opposite the large island, No. 
638 is the overlying rock. It contains many portions of a very hard trap, 
numerous thin veins of quartz, and many egg-shaped cavities filled with the same 
mineral, the quartz being surrounded with a coating of chalcedony. A narrow 
dike, bearing north and south, traverses the bedded rocks at this place. Just above 
the point is an exposure of amygdaloid, with beds of altered sandstone (No. 76) 
above it, and over them a bed of trap. Some portions of the exposure are earthy- 
looking, while other parts present every appearance of having been fused. A short 
distance above this place, the beds consist of metamorphosed sandstone, shale, and 
seams of clay, as illustrated in the following section : 
