The scene represented in the annexed plate (pl. 2), from a drawing by
Lieutenant Bowen, R. N., will enable the reader to comprehend the
incessant changes which the transport of boulders produces annually on
the low islands, shores, and bed of the St. Lawrence above Quebec. The
fundamental rocks at Richelieu Rapid, situated in lat. 46 degrees N., are
limestone and slate, which are seen at low-water to be covered with
boulders of granite. These boulders owe their spheroidal form chiefly to
weathering, or action of frost, which causes the surface to exfoliate
in concentric plates, so that all the more prominent angles are removed.
At the point _a_ is a cavity in the mud or sand of the beach, now filled
with water, which was occupied during the preceding winter (1835) by the
huge erratic _b_, a mass of granite, 70 tons' weight, found in the
spring following (1836) at a distance of several feet from its former
position. Many small islands are seen on the river, such as _c d_, which
afford still more striking proofs of the carrying and propelling power
of ice. These islets are never under water, yet every winter ice is
thrown upon them in such abundance, that it _packs_ to the height of 20,
and even 30 feet, bringing with it a continual supply of large stones or
boulders, and carrying away others; the greatest number being deposited,
according to Lieutenant Bowen, on the edge of deep water. On the island
_d_, on the left of the accompanying view, a lighthouse is represented,
consisting of a square wooden building, which having no other foundation
than the boulders, requires to be taken down every winter, and rebuilt
on the reopening of the river.

These effects of frost, which are so striking on the St. Lawrence above
Quebec, are by no means displayed on a smaller scale below that city,
where the gulf rises and falls with the tide. On the contrary; it is in
the estuary, between the latitudes 47 degrees and 49 degrees, that the
greatest quantity of gravel and boulders of large dimensions are carried
down annually towards the sea. Here the frost is so intense, that a
dense sheet of ice is formed at low water, which, on the rise of the
tide, is lifted up, broken, and thrown in heaps on the extensive shoals
which border the estuary. When the tide recedes, this packed ice is
exposed to a temperature sometimes 30 degrees below zero, which freezes
together all the loose pieces of ice, as well as the granitic and other
boulders. The