THEIR GEOGRAPHICAL CONDITIONS. 197 



viviparous, and, so far as this function of gestation is con- 

 cerned, hold an intermediate place, as it were, between birds 

 and the ordinary mammalia. Numerous teeth and jaws of 

 a small size (about that of a rat or rabbit) have been dis- 

 covered, some indicating herbivorous, some carnivorous, and 

 others insectivorous habits, but all apparently belonging to 

 the same pouch-bearing orders. It is true that in the chalk 

 marls some bones of a doubtfully higher character have been 

 detected, but beyond these fragments nothing higher in the 

 scale of Life than birds and marsupials is known to belong 

 to the secondary ages. 



Here then, during these secondary ages, which embrace a 

 long period of the world's history, we have seas of moderate 

 depth and broad shallow estuaries in which all the ordinary 

 sediments were deposited shelly and coralline limestones 

 in the outer waters, muds and clays in the stiller recesses, 

 and sands on the open shores. Here and there masses of 

 rock-salt and gypsum accumulate in detached lagoons and 

 sea-reaches j while during the oscillations of the land the 

 swamp and forest growths of a genial climate are repeatedly 

 submerged and converted into coal. We say genial climate, 

 for the cycads, zamias, palms, tree-ferns, and broad-leaved 

 pines (which are the prevailing forms), point to warm and 

 equable conditions ; while the frequent oscillations of the 

 land are amply shown in the numerous seams of coal and 

 "dirt-beds" or ancient soils on which the forest-growths 

 had flourished for ages.* And while these genial conditions 

 prevail over sea and land, both are equally exuberant with 



* One of the best known examples of an oolitic soil is the celebrated 

 " dirt-bed " of Portland an earthy, carbonaceous mass, replete with the 

 roots and prostrate trunks of cycads, zamias, and other vegetation charac- 

 teristic of and peculiar to those secondary formations. In fact, a genuine 

 forest-mould, with its rotted leaves, fallen trunks, an mbedded root- 

 stools. 



