10 J. L. LOBLEY — CRETACEOUS ROCKS OF ENGLAND. 



which inhabit northern and southern hititudes, leavinf? the inter- 

 mediate ocean as the habitat of the calcareous covered Foraminifera. 

 Those siliceous nodules, the flints, so abundant in the Clialk, re- 

 mind us that in the ancient waters there was in like maimer a large 

 amount of silica. It was formerly supposed that these marine 

 animals lived and died at the bottom. There now appears good 

 e\'idence for believing that they live near the surface, and that the 

 ooze is formed by the raining down as it were of the shells of the 

 dead animals from the upper stratum of the oceanic waters. This 

 is a fact of great importance in connexion with another now to be 

 mentioned. 



It has been ascertained that not merely is the calcareous white 

 ooze confined to certain latitudes, but it is confined to certain 

 depths also, and beyond those depths the white ooze is not to be 

 found. Between Africa and South America there is a sub-marine 

 valley of enormous depth, and at the bottom of tliis valley, instead 

 of the white calcareous deposit, there is a red argillaceous mud. 

 Now it has been found that about two per cent, of the material of 

 the Avhite ooze is argillaceous or clayey matter, and it is hence in- 

 ferred that this red mud is the argillaccHJUs portion of the Forami- 

 niferal remains which have rained down over this portion of the 

 Atlantic bed, but of which the calcareous portion has not reached 

 these exceptionally great depths, possibly in consequence of some 

 solvent agency in existence at certain depths. We thus learn that 

 even argillaceous deposits, clays and slates, may be produced by 

 organic agency, and the bold hypothesis of clay rocks generally, as 

 well as calcareous and siliceous, the limestones and the sandstones, 

 being of organic origin, has been enunciated by Professor Huxley. 



Judging of the past from what we know of the present, as is the 

 wont of geologists, we conclude that a wide-spreading and deep 

 sea has deposited the Chalk of England, and that its present ex- 

 tension by no means marks out the area over which it once spread. 

 The Wealden area has already been mentioned as ha\Tng been at 

 one time covered by the Chalk, and over districts now occupied by 

 sub-Cretaceous formations the great sheet of Chalk once extended, 

 but, yielding to the various wasting, or, to speak more correctly, 

 changing forces of Nature, it gradually diminished and ultimately 

 disappeared, leaving uncovered the rocks on which it had before 

 reposed. Lyell says, " Pure Chalk, of nearly uniform aspect and 

 composition, is met with in a north-west and south-east direction, 

 from the north of Ireland to the Crimea, a distance of about 1,140 

 geographical miles, and in an opposite direction it extends from the 

 south of Sweden to the south of Bordeaux, a distance of about 

 840 geographical miles." That the great Chalk bed continues 

 under districts where less ancient deposits occur, is abundantly 

 proved by well-sinking, and thus we know that though we do 

 not find th(^ Chalk at the surface between Watford and Loudon, it 

 (extends southwards from Watford under tlie great bi'd of London 

 Clay forming the Thames Valley, and risi-s again to the surface 

 near Croydon, in Surrey (see Fig. 5). Watford is, therefore, on 



