. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. xin.] Crag. THE NORWICH CRAG. Fig. 144. London Clay. 197 Chalk. chalk is explained in the foregoing diagram. Both the White and the Ked Crag, as we shall see in the sequel, belong to the Older Pliocene period, whereas a more modern deposit, occurring in the neighborhood of Norwich, is referable to the Newer Pliocene. It consists of beds of incoherent sand, loam, and gravel, which are ex- posed to view on both banks of the Yare near Norwich. As they contain a mixt
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Ch. xin.] Crag. THE NORWICH CRAG. Fig. 144. London Clay. 197 Chalk. chalk is explained in the foregoing diagram. Both the White and the Ked Crag, as we shall see in the sequel, belong to the Older Pliocene period, whereas a more modern deposit, occurring in the neighborhood of Norwich, is referable to the Newer Pliocene. It consists of beds of incoherent sand, loam, and gravel, which are ex- posed to view on both banks of the Yare near Norwich. As they contain a mixture of marine, land, and freshwater shells, with ichthy- olites and bones of mammalia, it is clear that these beds have been accumulated at the bottom of a sea near the mouth of a river. They form patches varying from 2 to 20 feet in thickness, resting on white chalk, and are covered by a dense mass of stratified flint-gravel. The surface of the chalk is often perforated to the depth of several inches by the Pholas crispata, each fossil shell still remaining at the bottom of its cylindrical cavity, now filled up with loose sand from the in- cumbent crag. This species of Pholas still exists, and drills the rocks between high and low water on the British coast. The most common shells of these strata, such as Fusus striatus, F. antiquus, Turritella communis, Cardium edule, and Cyprina islandica, are now abundant in the British seas; but with them are some extinct species, such as Nucula Cobboldice (fig. 145), and Tellina obliqua (fig. 146). JSfatica helicoides (fig. 147) is an example of a species formerly known only Fig. 145. Fig. 146. Fis. Nucula Cobboldia;. Tellina obliqua. ITaiica helicoides, Johnston. as fossil, but which has now been found living in our seas; and I have recently seen, in the British Museum, a living shell from Vancouver's Island, so closely allied to N. Cobboldice, that it would be considered by many as merely a marked variety of the same form. The Nor
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
