78 THE BRITISH MAMMALS ?AST AND PRESENT. 



from the Permian, our first mammal only from the Rhaetios, when 

 the sea broke into the Triassic lake which extended from what is 

 now the Continent, up the middle of England, into the north-east of 

 Ireland, and perhaps beyond. The sea extended the boundaries of 

 the lake, much as the Black Sea might do if it broke into the sinking 

 Caspian, and by the continued subsidence became deep enough for the 

 deposition of the Lias, and with many changes of level and other con- 

 ditions received on its bed that wonderful series of fossiliferous clays 

 and limestones we know as the Oolites ; the dark-coloured clays, like 

 the Lias, being probably due to the denudation of the surrounding 

 carboniferous country, the limestone forming when the water cleared, 

 and ceasing when each new arrival of clay destroyed its builders. 

 Eventually upheaval towards the north reduced the sea to the 

 smaller area in which the Portlandian series was formed, and further 

 upheaval left it as the lake, or series of lagoons, whose presence we 

 trace in the alternate freshwater, marine, and brackish fauna of the 

 Purbecks. 



When the Cretaceous period began, the land-surface extended 

 far westward and northward into the Atlantic, and the Purbeck 

 lagoons had become freshwater lakes which widened and united as 

 subsidence set in, until the rivers that fed them ran through into the 

 sea, which eventually spread over the entire area, and, after several 

 vicissitudes, became deep enough for the formation of the chalk, 

 when very little of what is now Britain remained above water as a 

 scattered group of islets in an ocean which stretched from Asia over 

 Europe, across the/ Atlantic into the southern states of North 

 America. 



After a long interval the chalk sea shallowed until much of its 

 bed appeared above the waters and became dry land, there being, 

 as usual, many outbursts of volcanic activity during this period of 

 elevation. The new land became clothed with luxuriant vegetation 

 characteristic of a warm climate, and in time was peopled by quite 

 a different fauna from that of the past. Then the waters began to 

 creep up in the south-east of what is now the British region, and in 

 the bed of the rising sea the Thanet Sands were laid down ; and 

 then, round the coast in the estuaries and lagoons, were formed the 

 Woolwich and Reading beds, the elevation of the Wealden dome 

 giving the island from the denudation of which came the materials 

 for the Oldhaven pebble beach, which, as the sea-bed further sank, 

 became the basement bed of the London Ciay. This familiar clay 

 was evidently deposited in the deeper parts of a shallow sea from 

 materials supplied by the surrounding rivers, one, or several, of 

 which must have drained a considerable extent of country. 



Towards the close of this clay period the sea again became 

 shallow, either by silting up or gentle upheaval, and much of the 

 London basin became a land surface in whose lakes and estuaries 

 our Bagshots were formed from the denudation of the country to 

 the north and west, and perhaps of the Wealden dome : while the 

 Hampshire basin became a gulf in which the beds of that area 

 were deposited from easterly-flowing rivers. The continued sub- 

 sidence brought in the sea from the west through the rocky high 

 ground between Cornwall and Brittany, the eastern and western seas 

 being kept apart for some time afterwards by the ridge in the south- 



