252 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



ozoic series. At the same time I must observe that it was impossible, 

 without actual experiment, to have arrived at this conclusion ; for 

 the Lower Greensand crops out with so much regularity in the dis- 

 tricts immediately south and north of London, and with characters so 

 much alike, that it was not possible to infer a priori the interruption 

 caused by the old underground ridge, nor can I even now believe 

 that there is a total want of continuity. It is evident that the Lower 

 Greensand must be continued underground to some unknown distance 

 between Reigate and London on the one side, and Woburn and Lon- 

 don on the other, and also that the Lower Greensand of Cambridge 

 must range for a certain distance in the direction of Ipswich. It is 

 a question of what was the size of the old palaeozoic land— was it a 

 ridge, with breaks in it at intervals, or a broad tract ? The first is, I 

 think, the more probable ; for I cannot imagine but that, from the 

 very peculiar mineral character of the mass in Bedfordshire and 

 Surrey, there must have been, in places, continuity between these 

 areas, and I therefore infer that the Lower Greensand may yet be 

 found under the Chalk at many places, and that, although not 

 immediately under the north of London, it yet will be found at no 

 great distance both to the north and south of that spot. 



Note. — Mr. BrufF has just sent me up a specimen of the slate (from 

 the depth of 1050 feet), containing an impression of what appears 

 to be a large Posidonia. If so, that will remove all doubt as to the age 

 of this rock, as this shell is, with the exception of one species which 

 occurs in the Lias, confined to the Palaeozoic rocks. — April 1858. 



3. On a Boulder of Granite found in the "White Chalk," 

 near Croydon ; and on the Extraneous Rocks from that 

 Formation. By Robert Godwin- Austen, Esq., F.R.S., G.S. 



Part I. CONTENTS. 



Introduction. 



Locality and Position of the Croydon Boulder. 

 Previous notices of extraneous materials in Chalk. 

 Description of the specimens exhibited. 

 Form of the Boulder. 

 Character and composition of the Boulder, and of the associated Sand and 



Greenstone. 

 Observations and Inferences. 



Part II. 



Mode of deposit, extent, and nature of the Fossils of the White Chalk. 

 Means of transport of extraneous materials in the Cretaceous Sea. 

 Form and composition of the Land of the Cretaceous Period. 



Part I. 



Introduction. — So recently as 1811, when Mr. James Parkinson 

 published his work on the ' Organic Remains of a Former World,' 

 petrifactions of animal- and plant-structures were called "adventi- 

 tious" or "extraneous," whilst all mineral materials, whatever their 

 character or the composition of the beds in which they occurred, 

 would have been considered as properly belonging to such beds and 

 in their natural positions. A true perception of the origin of the 



