324 Sir W. E. Logan on the Quebec Group, and 
group to the surface in the regions to the southeast, as already de- 
scribed in my letter to Mr. Barrande (this Journal, xxxi, 216) 
suggest the following considerations. 
rom the occurrence of wind-mark and ripple-mark on closely 
succeeding layers of the Potsdam sandstone, where it rests im- 
mediately upon the Laurentian series, we know that this arena- 
ceous portion of the formation must have been deposited imme- 
diately contiguous to the coast of the ancient Silurian sea, where 
ook of it was in some places exposed at the ebb of the tide. 
o want of conformity is known to exist between the Potsdam 
and Calciferous formations, and the Quebec group being of Cal- 
ciferous age and 7000 feet thick, it follows that during the Pots- 
dam period, while the sandstones of the formation were being 
deposited on a level with the surface of the sea, there must have 
existed a depth of at least 7000 feet of water over the area in 
which were subsequently deposited the strata of the Quebec group. 
ith the exception of a small mass of the Potsdam sandstone 
at St. Ambroise, we have no evidence of a marginal outcrop of 
this formation between the St. Maurice River and the Mingan 
Islands. No marginal outerops of the Calciferous and Chazy 
‘i from the longitude of Lake St. 
Peter to the same group of islands; and between the vicinity of 
e 
Lake Superior, where it appears without any diminution in 18 
volume, it 
From these facts, it would appear probable that, iu = 
Potsdam period, the older rocks, which formed the coast 
Lower Silurian sea, extended, under comparatively sh 
water, southeastwardly from the St, Lawrence and the sagt 
__ But although the great volume of the Quebec and Potsdam 
groups, shows that over the area occupied by them, there ist 
d a deep sea during the Potsdam period; _— 
