Chap. XX.] GENERAL SUCCESSION OF FORMER LIFE. 
487 
have lived at no great distance from the land of that time : and, in the suc- 
ceeding Carboniferous Period, when the Mountain-limestone was formed, 
we find no lack of floating shells and other Mollusca equally characteristic 
of certain depths of water ; and yet these are associated with abundance of 
Plants which were drifted from land, simply, as I would say, because the 
earth then bore a rich arboreal vegetation. In the earlier Silurian time, 
however, notwithstanding the certain existence of contiguous lands at that 
period, we can discover only feeble traces of vegetable tissue. In all the 
subsequent formations of the geological series, from the Permian up- 
wards, Land Plants ever accompany marine remains. Thus in the Lias 
and Oolites, for example, we meet with an equal, if not a greater number 
of floating shells than in the Silurian rocks (the Ammonites and Be- 
lemnites of that younger period, requiring the same depth of water, had 
taken the place of the primeval Orthocerata) ; and yet, associated with 
them, we have everywhere proofs of the proximity of the land, in the 
abundance of fossil Plants and wood derived from terra Jirma, doubtless 
then very much more extensive and diversified than in the earliest times. 
If the old continents and islands which existed during the accumulation 
of the Lower- Silurian deposits had borne trees, the numerous researches 
of geologists in all quarters of the globe must have brought to light some 
signs of them ; for, whilst we know that there are rocks of considerable 
extent which, from the fine nature of their materials, may probably have 
been deposited in an ocean at some distance from a shore (though we have 
as yet little or no evidence as to the accumulation of sediment in deep seas, 
where no currents prevail), there are, on the other hand, many Silurian 
districts of the Old and New World where the form and structure of the 
strata bespeak the action of waves and surge, and where the imbedded 
Sea-weeds, Zoophytes, and other remains compel us to adopt the same 
view. If, also, the primeval fauna does aflbrd fewer Spiral Univalves than 
are seen among the animals of the £ Laminarian Zone ' of modern seas, 
we may suggest that shore -lines, as we understand them, must have been 
much less numerous in primeval epochs than in the Tertiary period and at 
the present day, after the surface had been diversified by lofty dividing 
ridges on the land and corresponding depressions in the ocean. With this 
important reservation, however, we obtain as many of those signs of 
shores as we can expect to find in the earlier deposits. Appeal, for ex- 
ample, to some of the oldest sedimentary deposits in which traces of any 
living thing have been detected, the Longmynd rocks of Britain, and see, 
in their mode of aggregation, in the Sand-worms which burrowed into 
them, and in the rain-drops which dibbled their surface, the clearest proofs 
of shore-accumulation (p. 28). 
Again, look to the illustrations of this point furnished by the American 
geologists, from a very wide extent of their country, where the strata are 
nearly horizontal, and where, without any ambiguity, our kinsmen have 
