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THE QUESTION OF WORKABLE
would make this article unreadable to the general public. Such axes
of elevation as have been referred to are, however, but mere wrinkles
on the larger curvatures of the lithosphere of the globe ; while in
some cases, as in the broad arch of the Rocky Mountains, the later
stratified rocks still form a vast plateau, which may some day be
thrown into the form of a normal mountain-chain, when the deep
incisions made by the canons shall relieve the strain sufficiently to
allow the rocks below to be ridged up into another crystalline axis.
We must free our minds of the popular fallacy that the stratified
rocks of the globe are parts of what were once continuous layers
spread over the globe. All recent progress in geological science has
led to the recognition of the fact that they were definitely related to
the older continental masses, off the margins of which they were
deposited, from which also their materials were in great part derived
by the ordinary processes of waste and transport by water. This was
first, I believe, prominently put forward in this country by Professor
Geikie, in his presidential address to the Geographical Section of
the British Association at Edinburgh in 1892, though to some of us
who were familiar with the Continental literature of the subject, there
was nothing new in it.
Now the question naturally arises as to whether geology affords
any evidence of the stage in the earth's history at which "dry
land" was first formed by elevation above the once universal ocean.
This has been often considered and debated. For my own part
I am inclined to the view which I have put forward elsewhere,
and which is held by some geologists whose views are entitled to
respect, that we have no evidence of any general land elevation
before what is known as the Devonian age, when we meet with first
appearances of a definite land-vegetation, the anthracite and
graphite of the earlier strata being derived from marine Algae." All
the formations of previous date (the sandstones, the conglomerates,
the mudstones, the limestones, and the clays since altered into slates),
of the Cambrian and Silurian periods, are of the nature of marine
deposits, as testified by their fossil contents ; and where these fail
us, as in the Cambrian and Huronian conglomerates and sand-
stones (altered in many cases into quartzites), the occurrence of
such strata is perhaps best explained by the enormous tidal action
3. That is to say, the earlier stratified sedimentary rocks. The graphite of the Archaean rocks
had in all probability a different origin altogether, and had nothing to do with plants, but was of
mineral origin, as I have shown op. cit., and in a paper read at the Bath meeting of the British
Association, 1888.