File:The Quarterly journal of the Geological Society of London (12511796355).jpg

88 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Feb. 23,
bright green, in well-defined layers. It contains very numerous
nodules or fragments, chiefly of white translucent quartz, varying
from the size of a pin's head to that of a horse-bean. These are
cemented by a calcareous, opaque, friable matrix. On the west bank,
below the bridge, this sandstone has, both imbedded in it and resting
on the naked gneiss, large boulders of the same gneiss, — some weigh-
ing many hundred weight. One of these Sir Charles Lyell found by
admeasurement to be 8 feet long.
"When the water is low, an interesting fact is seen near the east
bank, 350 yards above the bridge. On the prolonged flank of a
mound of gneiss rest some horizontal layers of Potsdam sandstone.
They gradually thin ofi" as they ascend the mound and cease (see
fig. 1), but the superincumbent hmestone is continued, and, in im-
mediate contact with the gneiss mound, covers its little crest and its
opposite side.
Fig. 2. — Section showing the superposition of beds of Potsdam sand-
stone and Trenton limestone on gneiss. River Montmorenci.
a. Gneiss. h. Potsdam sandstone. e. Trenton limestone.
The Potsdam sandstone at this place has not yet been found to
contain organic remains.
The Trenton limestone, which reposes conformably on the Potsdam
sandstone, is amply displayed. It constitutes the whole mural gulley
at the Natural Steps, and lines both banks of the river either in slopes
or clifi's from thence to the brink of the great Falls. On the west
side of the brink the clifi" is 40 feet high, chiefly consisting of Trenton
limestone with the rocks beneath well exposed.
The Trenton limestone at Montmorenci is fetid, moderately hard,
finely granular, black, bluish, or blackish brown. Certain thin
layers at every level are abruptly crystalline and pale brown ; others
again take on a quasi-conglomerate appearance, with rounded con-
cretions, from the size of an orange downwards. At irregular intervals,
a dark calcareous shale, from 6 to 18 inches thick, is intercalated;
and this, as we shall find, is soon, under the name of Utica slate,
wholly substituted for the Trenton rock.
The sides of the clifi^ over which the Montmorenci plunges into
the River St. Lawrence show the manner in which the Trenton lime-
stone passes into the next and upper division of Silurian rocks, Utica
slate (represented in Wales by Llandeilo flags), and the Hudson
River group ; and it is thus : — on both sides of the chasm (the middle