Geology of Lake Superior. 21 
grey coloured rock in strata 10 or 12 feet thick. This rock 
alternates with thin strata of slate until near the summit, 
when there succeeds several conformable strata of sandstone. 
The whole of these strata were parallel, dipping to the south 
east at an angle of about 20° degrees with the horizon. The 
whole cliff was about 450 feet above the lake. We_ have lit- 
tle doubt that this dark rock and the alternating slate be- 
long to the greywacke formation, for we found a large mass 
of well defined greywacke on a neighbouring island. Lydian 
stone was found on tlie same island. It may not be unimpor- 
tant to remark here, that the greenstone of this lake has, in 
general, a massive and columiar structure at the summits of 
the cliffs, and that lower down those cliffs it becomes more or 
Tess slaty, in which it sometimes merely shows signs of strati- 
fication, in others, as at the Pic Island, it is decidedly so. It 
then sometimes contains globular concretions not unlike those 
observed in the greywacke of the St. Lawrence. We certain- 
ly feel some hesitation in opposing our opinion to that of Doc- 
tor Bigsby who conceives these appearances to arise merely 
from a change of structure in the greenstone. Nevertheless 
we aretlecidedin thinking, that, at least in the locality which 
we have best examined, the Pic island, the greenstone rests 
on a clay slate which we class in the submedial order, 
17. Before quitting the consideration of the sandstone, we 
must mention a singular rock which has been only slightly 
noticed by Doctor Bigsby. It occurs in cliffs on the north 
side of the easternmost Neepigon islands, where it is massive, 
of abrick red colour, striped and veined with impure white. 
It is surmounted by a single stratum of a dark grey rock which 
we could not getat, and which may be limestone. Doctor 
Bigsby found this rock near Thunder Cape, of a red colour, 
also massive, and superimposed on alternate strata of red 
sandstone and limestone destitute of organic remains. ‘This 
is the only instance of limestone in place hitherto discovered 
on 
