738 



GEOLOGY. 



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Z^L~-' (Cr ^ 

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than others. Of the numerous and remarkable family of crustaceans, the trilobites, 

 with which the seas of the Silurian epoch were crowded, not one survived the carboni- 

 ferous era. Tribes of enormous reptiles had appeared, swarming in the waters of estuaries, 

 or occupying the shores, all of which perished before the secondary period closed. Yet, 

 before it terminated, the highest form of animal organisation, the mammalian class 

 had been produced, though upon a very insignificant scale, the only vestiges of it being 

 a few minute jaws in one of the oolitic beds, belonging to the marsupial order. But, 

 as we advance through the tertiary system, we shall meet with numerous examples of 

 mammalia, of extinct species and genera in the older tertiaries, becoming more and more 

 identical with the living types in those of newer date. 



Before the publication by Cuvier and Brongniart of their memoir on the tertiary strata 

 around the city of Paris, in the year 1810, these beds were generally confounded with 

 recent superficial accumulations ; but, since that period, other similar deposits have been 

 studied with diligence and success, and the remains of animals imbedded in them of 

 genera and species distinct from the existing races, with the appearance of regular stra- 



tification, have led to their recognition as an 

 independent scheme of formations. The pre- 

 vailing materials of the system are arenaceous, 

 argillaceous, and calcareous, in general loosely 

 aggregated. They occupy great hollows of 

 the chalk, and occasionally of older strata ; and 

 owing to the tertiary beds having undergone 

 less pressure from their position at the surface 

 than more ancient deposits, their fossil shells 

 have been preserved with remarkable perfec- 

 tion. The arenaceous members are either 

 pebbly conglomerates, or sands, seldom indu- 

 rated $o as to form sandstone ; in some dis- 

 tricts white and colourless, in others of a 

 general green tinge, derived from the silicate 

 of iron ; while at Alum Bay, in the Isle of 

 Wight, the sand is of various hues, imparted 

 by the oxide of iron, giving to the rock a 

 fantastic, beautiful, and fairy-like appearance. 

 The argillaceous beds likewise exhibit great 

 diversity, being laminated clays almost pure, 

 or clays without lamination scarcely distin- 

 guishable but by their fossils from the allu- 

 vium of existing rivers, or clays with a sandy 

 admixture, or marls more or less calcareous. 

 Their colour varies from a dull blue or brown 

 tint to a light green ; but at Alum Bay the 

 hues of the associated clays are as diversified 

 as those of the sands, being almost black, and 

 again almost entirely red, or richly mottled 

 with red and white. The calcareous formations 



Marls with magnesite, m, near Coulommiers. 



are also very dissimilar, being rough coralline rocks, or coarse sandy limestones, soft, 

 marly, and full of shells, easily distinguishable from the limestones of the preceding systems 

 by their inferior degree of induration. The tertiary system includes subordinate formations 

 of lignite, or carbonised wood, forming an imperfect coal, which holds an intermediate place 



