18 Meyer — On the Cretaceous Rocks. 



partial submergence of tlie surrounding land-surfaces ; the character 

 of the fauna necessarily changing with the varying depth or altered 

 nature of the ocean-bed. 



I can scarcely therefore agree with those geologists who, judging 

 by the change of fauna, insist on the lapse of a long interval of time 

 in the passage from Lower Greensand to Grault. As well might we 

 require this lapse of time at the still greater change from Wealden 

 to Lower Greensand, the deposition of which is in places absolutely 

 continuous with the Wealden, and whose fauna we know to have 

 already existed to the southward long previously to its introduction 

 into the Wealden area by the intrusion of the Cretaceous ocean. 



And is it not, in like manner, equally possible that, previous to 

 the deposition of the Gault, much of its fauna may have been else- 

 where ah'eady in existence, as, for instance, in the Blackdown 

 Greensand ? The seemingly abrupt change of fauna from Lower 

 Greensand to Gault in our southern counties, being but . the natural 

 result of as sudden a change from a sandy to an argillaceous deposit. 



The strictly horizontal arrangement of the groups of strata shown 

 in the section, though true for short distances, is therefore, probably, 

 incorrect for each and all the groups, if traced throughout their ut- 

 most range, and must be regarded as merely an approximate arrange- 

 ment ; it being, probably, as true for sedimentary strata as for 

 forms of life, that all originated at some given point, and that, con- 

 sequently, their lateral extension can seldom be represented by a 

 horizontal line. 



EXPLANATION OF PLATE II. 



The right-liand portion of this section, extending from Folkestone, in Kent, 

 to Faruham, in Surrey, is taken along the range of the Xorth Downs ; its course 

 is consequently nearly parallel to the line of the South-Eastern Eailway, along 

 which, between Eeigate and Guildford, several of the lower sand-groups are well 



Vertical sections taken along this course, as at Nutfield or Guildford, would exhibit 

 such a succession of strata as is represented in the plate. 



The left-hand portion of the section follows a line nearly parallel to the above, but 

 is taken from the eastern coast of the Isle of "Wight to Lyme Eegis, in Dorsetshire. 



The names of the various groups are given to the right and left of the section ; 

 the changes in the mineralogical character of the beds, as from limestone to sand- 

 stone, or from argillaceous to arenaceous deposits, being in part indicated by 

 difference in strength of shading or other markings. 



The short vertical lines numbered 1 to 15, indicate the corresponding vertical 

 ranges of the several groups named below ; such of them as do not strictly fall within 

 the line of section having been introduced for the sake of comparison with the 

 others. 



I have also endeavoured to indicate on the section, by means of graduated vertical 

 lines, a roiigh outline of the distribution of the Cretaceous fauna which want of 

 space has prevented me from mentioning in the text. 



The graduated lines on the section must, therefore, be understood to represent the 

 vertical ranges of the fossils now contained in the several subdivisions of the Cretaceous 

 rocks. For as each of these subdivisions contains a group of fossils peculiar to itself, 

 and, also, some few species whose range is continued upwards through two or three more 

 lubdivisions, this difference in their vertical distribution is fairly represented by the 

 longer and shorter lines on the section. 



