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Some of these formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent.
The distance from the northern to the southern Downs is about 22 miles, and the thickness of the several formations is on an average about 1100 feet, as I am informed by Prof.
Nor is their rarity surprising, when we remember how large a proportion of the bones of tertiary mammals have been discovered either in caves or in lacustrine deposits; and that not a cave or true lacustrine bed is known belonging to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record mainly results from another and more important cause than any of the foregoing; namely, from the several formations being separated from each other by wide intervals of time.
But we can, I think, see why the geological formations of each region are almost invariably intermittent; that is, have not followed each other in close sequence.
I am convinced that all our ancient formations, which are rich in fossils, have thus been formed during subsidence.
Consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation, may have been formed over wide spaces during periods of subsidence, but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay.
It would seem that each separate formation, like the whole pile of formations in any country, has generally been intermittent in its accumulation.
Why then is not every geological formation and every stratum full of such intermediate links?
The many cases on record of a formation conformably covered, after an enormous interval of time, by another and later formation, without the underlying bed having suffered in the interval any wear and tear, seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition.
And now, before they could even assume a proper formation for defence, the outflanking /Impis/ had leapt, like greyhounds, on their flanks.
They are certainly anterior to the formation of craters and circles, for several have introduced themselves by breaking through their circular ramparts.