Dr. FiTTON on the Strata below the Chalk, 32b 



inference can be drawn with respect to the continuity beneath them of the 

 Wealden and Portland strata. It is not improbable that what now appear to 

 be detached portions, may in many cases be united ; but it seems to be con- 

 sistent with the mode in which freshwater deposits have been formed^ whe- 

 ther in lakes or estuaries, that they should, in more extensive regions, occur in 

 detached^ rather than continuous portions; and that the outlines of the spaces 

 which they occupy should be irregular. The former existence of the upper 

 Wealden strata in the interior of England, is rendered probable by the ero- 

 sion of the Purbeck beds in many places where the Lower green-sand comes 

 into contact with them, (131.) p. 286, — which has clearly been effected by 

 the action of water, and proves that something was carried away, before the 

 sand was deposited : the argument from these appearances being the same 

 as that derived from the erosions on the surface of the Oxford oolite and of 

 the chalk, (143.) p. 274. 



The great extent of the surface occupied by water in North America is a 

 striking feature in every ordinary map. Dr. Richardson, in his observations on 

 the geology of that region, has remarked that a large tract near the confines 

 of the primitive and secondary regions is occupied by lakes of the most varied 

 outlines and dimensions; — in which the deposition of shells and other exuvia: 

 of organized beings has been going on ever since the surface of the globe 

 assumed its present aspect. If that part of the northern hemisphere were 

 now to be sunk beneath the sea, we should probably have, after a short 

 time, one universal sheet of marine strata, lodged upon the surface of the 

 present land, so as to conceal not only the deposits of the several lakes, but 

 the intervals between them : and if we could, then, examine the internal 

 structure of the tract, we should find it to be occupied by a numerous series 

 of Wealdens, detached from each other, and bordered, it is more than pro- 

 bable, by deposits analogous to those of the dirt-beds of Portland, wherever 

 the fluctuations of the lacustrine waters left time and space for the growth of 

 plants upon their margin. The external boundaries of these deposits would 

 be as various and irregular as that of the lakes which now diversify the map 

 of North America: and we should expect, in such cases, that the mineral 

 components in the deposits of the lakes, would vary according to the nature 

 of the surrounding land. Their fossil contents would, probably, have a 

 great general resemblance throughout large districts ; but, in the remoter 

 tracts would differ as much as the productions of modern lakes are found 

 to vary ; — as those of the lakes in the North of England differ from those 

 of Switzerland, and both from those of the South of France, &c. ; or, as 

 in America, those of Slave-Lake differ from the productions of Lake-Superior 



