BEAVER BAY GROUP. 299 
much difficulty in determining the positions which these rocks occupy. 
Some of these places are difficult because, between the porphyries and the 
nearest bedded basic rocks, there are gaps without exposure, such as shingle 
beaches along the coast, and these gaps are either so long that the relations 
of the rocks can only be guessed at, or else they are so short that it is hard 
to see how one rock can pass under the other. So many of these cases 
proved tractable under further study that there remains but little doubt that 
in nearly all the porphyry would be found plainly enough to overlie or 
underlie the other rocks in a regular way. In yet other cases, however, 
vertical contacts between the two rocks were found, and while these con- 
tacts are in some cases demonstrably due to faulting, in others this is 
not so evident, and there are certainly places where the coarse-grained 
basic rocks look much as if intrusive, while the red granite-like rocks 
always present this appearance. Undoubted dikes occur cutting the por- 
phyries, and that much more frequently than in any of the preceding 
groups, but they are always of fine-grained rocks, unlike the associated 
coarse-grained kinds. The aphanitic, brown and gray rocks above referred 
to, as well as the rare amygdaloids and associated fine-grained diabases, are 
always plainly bedded. No rocks distinctly of detrital origin were observed 
in the group. 
On the whole, the group may be briefly described as made up of bedded 
coarse basic rocks, with interbedded fine-grained basic rocks, only rarely 
amygdaloidal, and also of interbedded acid porphyries in very irregular 
areas, which are individually limited in extent, contracting suddenly from 
several hundred feet in thickness to nothing. The whole group is much 
faulted; fine-grained diabase dikes are not uncommon, while gabbros and 
coarse-grained granite-like rocks are present in intersecting masses. The 
total thickness is probably understated at 6,000 feet. 
In describing this group more in detail, I find it most convenient to fol- 
low its exposures from west to east along the coast, taking the different kinds 
of rocks as they come. Beginning on the west, the basal bed of the group is 
a felsite, forming the bold point one and one-half miles above the mouth of 
Split Rock River. For some distance both east and west of the mouth of this 
river the rocks trend far around to the north, west of the river lying nearly 
