338 Prof. Sedgtoickonthe [May, 



Every one who is acquainted with the continental works on 

 geology must have remarked the extraordinary meagerness of 

 their details respecting most of the secondary rocks which are 

 newer than the formations of the Th'urinfrcrwald. The English 

 formations belonging to the same epoch are exhibited in a beau- 

 tiful and perfect order, for which it is in vain to look in any 

 other part of the world which has been yet examined. The 

 great beds of clay, interposed between the several oolitic depo- 

 sits, first enabled Mr. Smith to separate them from each other, 

 and to trace their distinctive characters. All such points of 

 detail, when once well established, become so many new terms 

 of comparison, by which we may eventually be enabled to fix 

 the relations between our own formations, and those in other 

 parts of the world. An accurate delineation of all those argilla- 

 ceous beds, which form so distinguishing a feature in English 

 geology, is certainly an object of the first importance. Our geo- 

 logical maps, as far as they are constructed on that principle, 

 not only point out the demarcation between mineral beds of 

 separate characters, but acquaint us at once with the general 

 aspect of the surface. The colours of the argillaceous beds 

 representing the extent of low, marshy, featureless districts ; 

 while the colours of the several oolitic formations, and of the 

 chalk, point out the limits of a succession of bold escarpments 

 and prolonged natural terraces. 



The preceding digTession has not originated in any spirit of 

 captious criticism ; but in difficulties which have been expe- 

 rienced in classifying some of the beds which support the chalk; 

 and in an earnest wish that the authors of our geological maps, 

 who have supplied us with so many admirable details, may leave 

 nothing undone which can possibly be effected. 



V. On the Formations of Sand and Plastic Clay, 

 London Clay, 8cc. 



The English formations which rest immediately upon the 

 chalk belong to a distinct epoch in the natural history of the 

 earth ; for they are not co-extensive with, nor always conforma- 

 ble toj the beds by which they are supported, but rather resemble 

 materials which have been mechanically drifted into the deep 

 dispressions or water-worn denudations of the older rocks. They 

 are, therefore, generally limited to the extent of certain pre-exist- 

 ing inequalities in the surface of the globe. 



Deposits originating in the way we have described must 

 necessarily be of variable thickness, and liable to every possible 

 modification from the action of mere local causes. Any useful 

 classification of their component beds would, perhaps, never 

 have been effected, had not the organic remains preserved in 

 them exhibited an extraordinary uniformity of character and 

 arrangement. An accurate examination of these spoils has, 

 therefore, supplied us with the means of establishing analogies 



