122 SKETCH OF THE HISTORY Or MAMMALIA. 



globe requisite for the existence of animals and plants 

 were not then established.* How long did this state of 

 the earth continue ? It is a question which cannot be 

 answered. Passing from the primeval rocks, 



" Where the bird dared not build, nor insect wing 

 Flit o'er the herbless granite," 



we come to the Snowdon rocks of argillaceous slate, and 

 the calcareous and argillaceous rocks, constituting the 

 Cumbrian system, in which a few traces of organic life 

 have been detected, but of organic life in its lowest type. 

 Then the Silurian or transition system succeeds, con- 

 sisting of sandstones, limestones, and shale ; here corals, 

 crinoidea, trilobites, terebratulae, &c., all belonging to 

 extinct species, and often to extinct genera and families, 

 and all aquatic, are abundant. From these systems of 

 the primary strata, we advance to the secondary strata — 

 rich in oceanic life — divided into the carboniferous 

 system, the saliferous or new red-sandstone system, the 

 oolitic system, and the cretaceous or chalk system. The 

 deposits constituting each of these systems are replete 

 with organic remains, but all of extinct species and 

 often of extinct genera. The coal-measures are rich in 

 an extinct Flora, principally consisting of ferns, often in 

 an extraordinary degree of preservation, the most deli- 

 cate leaves being spread out, and so arranged as to con- 

 stitute a beautiful Hortus Siccus of a long-past period. 

 About 300 species of plants have been discovered in the 

 coal-measures of this and other countries. Their luxu- 

 riance indicates a genial temperature and a humid ground. 

 "It would hardly be credited," says Professor Lindley, 

 in his 'Fossil Flora of Great Britain,' "by persons un- 

 acquainted with the evidence upon which such facts re- 

 pose, that in the most dreary and desolate regions of the 

 present day there once flourished groves of tropical 

 plants, of Coniferse, like the Norfolk Island and Arau- 



. * We exclude microscopic animalcules from our consi- 

 deration, because at present we scarcely know under what 

 circumstances they can live. 



