688 A. C. LANE AND A. E. SEAMAN 
Longmyndian flag stones and 
shales and Rushton schists imukie black slate.and 
graywackes. 
the correlates to the Hu- 
ronian cherts and 
jaspilites. 
g correlates to the An- 
Bradgate, Beacon Hill and 
Black Brook hornstones of 
Charnwood Forest 
The Keweenawan would then indeed be pre-Cambrian. But far-off 
lithological correlations have too often proven a will-of-the-wisp. 
Lower Keweenawan.—The conglomerates which occur at the top 
of the Lower Keweenawan and below the Lake Shore traps may 
perhaps belong stratigraphically to the Upper Keweenawan. They 
contain a wide variety of pebbles and large bowlders, and are in 
structure at times suggestive of till. 
The main part of this formation cannot at its thinnest point be 
put down as much less than 15,000 feet. Its base in contact with 
the upper beds of the Animikie formation has recently (H. L. 
Smyth) been found north of the Gogebic range. Here its thick- 
ness appears to be near 42,500 feet, and I do not think it can 
possibly be reduced by any allowance for repetition by faulting and 
initial dip to less than 29,000 feet. Some such thickness of rock 
with the specific gravity of trap over against an equal thickness of 
granitic rocks would be needed to produce isostatic equilibrium 
between the bottom of Lake Superior and the Huron Moun- 
tains. Of this thickness only a small fraction is sedimentary 
and that of a type which may be rapidly accumulated, so that 
the whole series is not necessarily of great duration. The gen- 
eral characteristic of the lavas which made it up is that, while they run 
from silicious felsites and quartz porphyries to quite basic rocks, 
there is nothing ultra basic nor ultra alkaline known. Porphyritic 
crystals: are mainly quartz and feldspar. These have normally 
crystallized earlier in the magma. A marked second genera- 
tion of augite is almost unknown. When we come to estimate how 
much of it is sediment, however, we find that continuous diamond- 
drill sections would indicate for the major part of it less than 7 per 
cent. (5.65 per cent. in 8,500 feet, 6.65 per cent. in the 6,247 feet at 
Isle Royale). The greater the thickness probably the less the per 
centage. Making detailed allowance for the Great and certain other 
cee ee we 
