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No less conclusive is the presence of sea-laid rocks which we may find in the neighboring quarry or outcrop, although it may have been long ages since they were lifted from the sea to form part of the dry land. Among common proofs of subsidence are roads and buildings and other works of man, and vegetal growths and deposits, such as forest grounds and peat beds, now submerged beneath the sea.
If the flow occurred from some submarine volcano, we may recognize the fact by the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers.
Such markings may be covered over with a thin layer of sediment at the next flood tide and sealed away as a lasting record of the manner and place in which the strata were laid. STRATIFICATION. For the most part the sheet of sea-laid waste is hidden from our sight. Where its edge is exposed along the shore we may see the surface markings which have just been noticed.
It joined with Lake Ontario, and extending down the Champlain and Hudson valleys, made an island of New England and the maritime provinces of Canada. The proofs of this subsidence are found in old sea beaches and sea-laid clays resting on Wisconsin till.
BASAL CONGOLMERATES. As the sea marches across the land during a slow submergence, the platform is covered with sheets of sea-laid sediments. Lowest of these is a conglomerate, the bowlder and pebble beach, widened indefinitely by the retreat of the cliffs at whose base it was formed, and preserved by the finer deposits laid upon it in the constantly deepening water as the land subsides.
Both a and b rise to a common level nn and upon them rest the horizontal sea-laid strata c, in which the upper portion of the canyon has been cut. Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down, mm' or nm?
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