THE STKUCTUKE AND CLASSIFICATION OF ROCKS. 



641 



CRETACEOUS PERIOD. 



Few examples of land organisms. Plants rare, 

 and nearly all referrible to marine types. 

 Sponges, zoophytes, star-fishes, echinites, and 

 molluscs abundant. Fishes of the ctenoid 

 and cycloid orders, to which the perch, 

 herring, and nearly all existing fishes belong, 

 for the first time appear. Keptiles common, 

 identical with, or closely allied to, those of the 

 Turrilites Costatus. Mocesamus Concentricus. prece( ji n g era . Bi r( } s ^fl m0 nkeys indicated. 



TERTIARY PERIOD. 



The prevalence of mammalian life is the grand distinguish 

 ing feature of this epoch. All the existing orders are 

 represented, except man, though by extinct species, 

 many of which did not appear till towards the close of 

 the era. The most remarkable are of the order of 

 toothless animals, as the Megatherium (great -wild 

 beast), the Megalonyx (great claw), and the Glyptodon 

 (sculptured tooth), gigantic analogues of the sloth, 

 armadillo, and ant-eater. The thick-skinned order is 

 represented by the Palceotherium (ancient wild beast), 

 the Anoplotherium (defenceless wild beast), the Deino- 

 fherium, (terrible wild beast), uncouth, tapir-like forms ; 

 and by allies of the rhinoceros and hippopotamus, with 

 true elephantoid genera, as the Mastodon and 

 Mammoth. 



POST-TERTIARY PERIOD. 



Remains of plants and animals belonging to species now 

 existing, more or less fossilised. 



The manner in which organic remains have been preserved varies with the nature of the 

 deposit in which they are embedded. In the more recent rocks, the entire skeletons of 

 animals have been found, which have scarcely undergone any alteration, but more 

 frequently the harder parts alone occur, and with these the teeth are met with most 

 abundantly, still retaining their sharp edges and enamel. Shells also are common in 

 strata of very high antiquity, as perfect in form as if newly picked up on the shore of the 

 ocean ; and microscopic animals, which the slightest disturbance must have crushed, 

 appear uninjured, which proves that they must have lived and died on or near the spot 

 where they are found, have been deposited at the bottom of very quiet waters, and that 

 the consolidation of the strata was a very gradual and uninterrupted process. Dr. Mantell 

 remarks, that in his early researches he fell into the error of supposing that all fossils 

 must necessarily be petrifactions, and threw away many beautiful shells that were associated 

 with ammonites in the marl at Hornsey, supposing, from their perfect state, that they had 

 been accidentally imbedded, and were not genuine fossils. " But the state of preservation," 

 he adds, " and the degree of change which an organic body has undergone in the mineral 

 kingdom, have no necessary relation to its antiquity. The shells in some of the ancient 

 secondary strata are frequently as perfect as those of modern tertiary deposits. I have 

 collected, in the lowermost clays of the Wealden, fresh-water shells having traces of the 



