MINERALS AND GEOLOGY OF CANADA. 44:7 
have come into their present positions, not by the sinking and re- 
tirmg of the sea, but by the actual elevation of the land. 
Many strata afford proofs of having been elevated and depressed 
above and beneath the sea, successively, at different intervals. Many 
sandstones, for example, exhibit ripple-marked surfaces, and some, 
impressions of reptilian and other tracks, through their entire thick- 
ness. This indicates plainly that they were formed slowly in shallow 
water, and that they were left dry, or nearly so, between the tides. 
And it indicates, further, that the shore on which they were deposited 
layer by layer, was undergoing a slow and continual movement of 
depression, otherwise the process of formation would necessarily have 
ceased, and the strata would present a thickness only of a few inches, 
or of a few feet at most. Afterwards a period of upheaval must have 
commenced, bringing up the rocks to their present level. In certain 
strata, also, the upright stems of fossil trees occur at various levels ; 
and in some localities, beds containing marine fossils are overlaid by 
others holding lacustrine or fresh-water forms; and these again by 
others with marine remains. Finally, to bring this section to a close, 
we have a striking example of alternations of land-upheaval and 
depression in the geology of Canada generally. Around Toronto, for 
example, we have strata of very ancient formation, belonging to the 
Lower Silurian series, overlaid by deposits of clay, gravel, and sand. 
Between the two, a vast break in the geological scale occurs. In 
other parts of this continent, many intervening formations are pre- 
sent (see the Table of Rock Groups, a few pages further on); and 
hence, it is concluded, that the Silurian deposits of this locality, after 
their elevation above the sea, remained dry land for many ages, whilst 
the intervening groups were under process of deposition in other 
spots ; and that, finally, at the commencement of the Drift period, 
the country was again depressed beneath the ocean, and covered with 
the clays, sands, and boulders of this latter time. Another period of 
elevation must then have succeeded, bringing up both the Silurian 
and the Drift formations to their present levels above the sea. 
(6) Denudation.—This term, in its geological employment, signifies 
the removal or partial remeval of rock masses by the agency of water. 
The abrading action of the sea, of rivers, &c., acting under ordinary 
conditions, has already been alluded to; but the erosive effects of 
water, under conditions now no longer existing, may be seen in nu- 
merous localities. Sections of the kind shewn in the accompanying 
