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CHAPTER XV. 



ON THE FORMATION OF SECONDARY LIMESTONE AND SANDSTONE, 

 AND ON THE PROGRESSIVE DEVELOPMENT OF ORGANIC LIFE. 



On the Decomposition of Chalk. — Whether formed by Animal Secretion, or by 

 Eruptions of Water holding calcareous Earth in Suspension or Solution. — Mud 

 Volcanoes.— Animal Bodies suddenly encased in Chalkindicatethe Timerequir- 

 ed to form a Stratum of a given Thickness. — Oolite and Encrinal Limestone 

 partly formed by Animal Secretion. — Formation of Sandstone.— Repeated Ap- 

 pearance of Dry Land during the Epoch when the Secondary Strata were de- 

 posited. — Progressive Development of Organic Life in the Secondary and Ter- 

 tiary Epochs. — Disappearance of enormous Reptiles and chambered Shells from 

 the Seas of Northen Latitudes. — Probability of the Icthyosaurus existing, as a 

 living Species, in the present Seas. 



Having travelled with the reader, over the secondary strata, from 

 the lowest new red sandstone, to the upper chalk, he may not be dis- 

 inclined to pause awhile, and look back upon the ground which he 

 has already passed, comprising a series of calcareous, sandy, and 

 argillaceous beds, whose aggregate thickness may not be less than 

 ten thousand feet. It is scarcely possible, in observing these beds, 

 and the bones and shells of extraordinary animals which they con- 

 tain, not to feel some desire to ascertain the causes by which they 

 were thus entombed, and to enquire in what manner, or by what 

 agents, the different beds were deposited or consolidated. Such 

 researches form rational and legitimate subjects for the meditation 

 of the geologist, though he may frequently have to lament the imper- 

 fection of his present knowledge, and the mystery in which many of 

 the processes of nature are still involved. 



One of the most ancient geological enquiries relates to the forma- 

 tion of limestone rocks and strata. Whence was the calcareous mat- 

 ter derived^ Some limestone rocks are composed chiefly of shells, 

 or other calcareous remains of marine animals, and in such instan- 

 ces we can have little hesitation in ascribing their formation to animal 

 secretion, similar to what is taking place in the numerous coral reefs 

 in the Pacific ocean. There are other beds, however, such as chalk, 

 to which a similar formation cannot be ascribed ; for though they 

 contain numerous organic fossils, these do not bear the proportion of 

 one to one hundred millions, when compared to the whole mass, and 

 the chalk does not appear to have undergone any chemical change, 

 from heat or other causes, that could have obliterated the traces of 

 organic existence. In no formation are the most delicate organic 

 textures of animals better preserved. In Mr. Mantell's splendid 

 collection of chalk fossils at Lewes, there are specimens of fish, in 

 which the body is entire and the air-bladder is uncompressed — and 



