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It is, I believe, generally accepted by the geologists that the accumulation of much of the sediments of the Cambrian, Carboniferous, Permian, Triassic, Jurassic and Cretaceous periods took place in shallow water, and that the sea bottom slowly sank under the weight of the increasing deposits.
There may have been as much matter in a state of vital organization in Carboniferous or in Cretaceous times as in our own, but there is certainly more now than in early Palaeozoic times. Yet every grain of this matter has existed somewhere in some form for all time.
It is demonstrable that an animal of the cretaceous sea might die, that its skeleton might lie uncovered upon the sea-bottom long enough to lose all its outward coverings and appendages by putrefaction; and that, after this had happened, another animal might attach itself to the dead and naked skeleton, might grow to maturity, and might itself die before the calcareous mud had buried the whole.
Even without the aid of organic remains we might have decided that the deposit was newer than the chalk, for chalk- flints containing cretaceous fossils were detected by the duke in the principal mass of volcanic ashes or tuff. These interesting discoveries in Mull led geologists to suspect that the basalt of Antrim, in Ireland, and of the celebrated Giant's Causeway, might be of the same age.
The grasses, in particular, do not appear until the Cretaceous, and do not show much development until the mid-Tertiary; and their development seems to be chiefly connected with physical conditions.
In Cretaceous times both the subsidence and the lava- flows seem to have been worldwide. We seem to think that the earth has sown all her wild oats, that her riotous youth is far behind her, and that she is now passing into a serene old age.
The number of exotic forms which are common to the Eocene and Miocene strata of Europe, like those to be alluded to in the sequel which are common to the Eocene and Cretaceous fauna, demonstrate the remoteness of the times in which the geographical distribution of living plants originated.
That this crushing was done before the salt waters retired from the region, which was towards the close of the Cretaceous Age, is sufficiently evident in the fact that portions of the Red Beds show similar fractures with the cracks filled with gypsum, and gypsum, as we have already seen, is a salt water deposit.
That of a genus allied to the Diplodocus measures six feet and eight inches, and the total length of the animal must have been not far from eighty feet, the largest land animal known. The Sauropoda became extinct when their haunts along the rivers and lakes of the western plains of Jurassic times were invaded by the Cretaceous interior sea. We may notice only two of the most interesting types.
Cassian beds, for example, Palaeozoic and Mesozoic forms are commingled, and, between the Cretaceous and the Eocene formations, there are similar transitional beds. On the other hand, in the middle of the Silurian series, extensive unconformity of the strata indicates the lapse of vast intervals of time between the deposit of successive beds, without any corresponding change in the Fauna.
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