PORTAGE LAKE SECTION. 193 
dark sandstone belonging near the same horizon in the quarry on the north 
side of Portage Lake, two miles below Hancock; the sandstone on the Lake 
Superior coast, near the mouth of the ship-canal, and a few other small and 
indefinite sandstone exposures, are all that have come to my knowledge as 
occurring anywhere near the line of section now under description. That 
the whole distance between the upper limit of the Lower Division and the 
main lake coast is occupied by sandstone, at all events with not more than 
a very few subordinate layers of diabase near the base of the Upper Divis- 
ion, is not only inferred from the topography, and from all that can be 
learned of the Upper Division of the series farther west, where sections 
across most of its thickness can be seen, but also seems to be proved by 
the exposures along the coast of Keweenaw Point, north of the ship-canal 
entry. 
The black shale and associated sandstone occurring west of Hancock 
are of peculiar interest because they unquestionably mark the same horizon 
as that of the black shale and sandstone of the Porcupine Mountains, and 
thence many miles both to the east and west. They vary from light-gray, 
rather fine-grained sandstones, to nearly black shales and shaly sandstones. 
They are closely associated with and even interstratified with coarser and 
redder kinds, which are yet of a darker shade than the usual red sandstone. 
All appear to consist chiefly of the usual porphyry detritus, but the par- 
ticles are predominatingly either the originally porphyritic feldspar or quartz, 
particles of matrix being subordinate. There is also in every section more 
or less basic detritus, in the shape of rounded particles of magnetite and 
augite, the latter often much altered to a greenish substance, and particles 
of the basic rocks showing the several ingredients together. 
On the lake shore, a mile below the entrance to the ship-canal, Sec. 29, 
T. 56, R. 34 W., begins a long line of low cliffs of sandstone, which ex- 
tend westward for some miles. These sandstone layers dip lakeward about 
8°, and strike more to the southward than the lake coast, so that higher 
layers come in to the westward, in which direction also the dip continues to 
flatten. The rock seen on section 29 is mostly red shale, but there are 
heavier layers of relatively coarse sandstone, sometimes light-gray, some- 
13LSs 
