138 BRITISH LACUSTRINE STRATA. 



calcareous deposits were formed because the sheets of fresh water rested 

 chiefly upon limestones, but the limestones alternate with marls indica- 

 tive of small streams which brought mud from time to time into the 

 waters. Mr. God win -Austen, with the best evidence in support of 

 his view, has taught us to regard the overlying Wealden formation, 

 which is scarcely divided from the Purbeck stratigraphically, as another 

 lacustrine deposit. But the physical geography of Europe had in the 

 meantime somewhat changed, and the Wealden lakes accumulated 

 chiefly, alternations of sandstones and clays with thin beds of fresh- 

 water limestone at intervals, and now and then an oyster shell, show- 

 ing that the waters of the sea may sometimes have communicated 

 with the great inland basin. 



Tertiary deposits in the Woolwich and Reading series certainly 

 make us acquainted with a lacustrine formation, which covered up 

 with freshwater shells the leaves of trees and deciduous vegetation 

 which grew on the lake banks. But this was a lake on much the 

 same level as the sea, with the fresh water passing insensibly through 

 brackish conditions to the sea, and giving evidence that sharks and 

 other marine fish penetrated as close up to the freshwater lake as 

 they do at the present day in some of the inlets on the western side 

 of Scotland. After the elevation towards the close of the London 

 Clay period, which is sufficiently evidenced by the multitude of fruits 

 in which the clay abounds, upheaval culminated in the formation of 

 the Lower Bagshot Sands, which appear to indicate lacustrine condi- 

 tions ; for not only are the sands filled with leaves of plants wherever 

 there is a bed of clay in which these spoils of a luxuriant vegetation 

 could be preserved, but the clay itself is, without exception, beautiful 

 white pipe-clay, which could hardly have been the case had it been 

 deposited from the waters of a great river necessarily charged with 

 iron salts from the decay of the crystalline rocks, and with carbonic 

 acid from the decay of vegetation such as is preserved. Finally, 

 with the so-called freshwater strata of the Isle of Wight we enter 

 on a period of time during which upheaval of the sea-bed took 

 place, and lakes and lacustrine deposits became general in Western 

 Europe, with occasional alternations of sea and land. The Headon 

 beds, Osborne and Bembridge beds, and the Hempstead beds give us a 

 scarcely broken sequence of old land-surfaces in the south of England, 

 on which plants of varying kind succeeded each other, and ever-new 

 types of mammals came down to the waters to die. The lakes on 

 this ancient land, best evidenced by the limestones, persisted, some- 

 times changing their outlines and altering the character of the deposits 

 formed in them, occasionally opening to the sea, and sometimes 

 receiving new elements in their fauna. 



Valleys. A valley is a long depression or hollow on the surface 

 of the earth, margined by ground more or less high. It may be 

 broad and shallow, or narrow and deep. It may be surrounded by 

 hills, or run through a country from sea to sea, entirely unassociated 

 with mountains. On a large scale valleys are exemplified by the great 

 depression which is filled with the Atlantic Ocean ; on a small scale 



