Vol. 50.] BORINGS AT CT/LFORD, WINKFIELI), WAKE, AND' CHESHENT. 493 



Slides were also made from the greenish slate, in view of the 

 possibility of its containing minute organic remains, but though 

 these slides were examined by Mr. Teall, by Messrs. Sharman and 

 Newton, as well as by ourselves, not the slightest trace of any 

 organic structure has been detected. 



Classification of the Beds. 



The conclusions to be drawn from the above description seem to 

 be as follows : — 



(i) If the sample from between 373 and 389 feet is from the base 

 of the Melbourn Rock, the beds below 389 feet belong to the 

 Lower Chalk. Samples 2 and 3 must in this case have been taken 

 from chalk and flints which fell down the bore. No sample of 

 Totternhoe Stone or of the sandy base of the Chalk Marl has been 

 preserved. 



(ii) Only one sample in 79 feet of strata between the depths of 

 483 and 562 feet has been kept, and this proves to be Gault. We 

 are therefore without evidence as to the base of the Chalk, but if 

 the thicknesses or Lower Chalk and of Gault are similar to those 

 near Burwell and Soham, west of Culford, there would be about 

 150 feet of the former and 90 of the latter, or 240 feet altogether. 

 Now, assuming that the Lower Chalk in the boring begins at 389 

 feet and that the Gault ends at 625 feet, these two formations are 

 236 feet thick, which is a very close correspondence. 



There is, however, some little doubt as to the base of the Gault, 

 for the sample from 600 feet had grains of coarse sand such as 

 generally occur only in the lowest 5 or 6 feet of the Gault, and it 

 may be therefore that the actual base of that division is very little 

 below 600 feet; the rest of what is termed " sandy Gault" being 

 similar to that in the underlying beds described as " sandy Gault 

 and stone." 



As the Gault is likely to be somewhat thinner towards the east, 

 we do not suppose that more than 30 out of the 79 feet called 

 " grey chalk-marl " is really Gault. If this inference be correct, 

 the Lower Chalk at Culford is 143 feet thick, its base occurring at 

 the depth of 532 feet ; while if the base of the Gault be taken at 

 605 feet, this division would have a thickness of 73 feet. 



With respect to the beds between Q25 and 636 feet, the brown 

 sandstone enclosing lignite has the aspect of ordinary quartzose 

 Lower Greensand, but the calcareous beds do not resemble any 

 part of that group exposed in Cambridgeshire or Norfolk. These 

 limestones were in fact so different from any rock previously known 

 to us that we were long in doubt about their age, and the possibility 

 of their being Jurassic had occurred to us ; but on referring to 

 Prof. Judd's account of the rocks below the Gault at Richmond l 

 we recognized a certain similarity of structure. On mentioning 

 this to Prof. Judd he very kindly lent us some of the slides which 



1 Quart. Journ. Geol. Soc. vol. xl. (1884) p. 738. 

 Q. J. G. S. No. 199. 2 l 



