xlii 
PROCEEDINGS OE THE 
beds running about half-way between Totternboe Knoll and Stan- 
bridge Ford Station, and following, at a distance of from balf a mile 
to a mile, the first rise of the north-western flank of the hills they 
were on, from Houghton Regis to Totternhoe, and again past 
Ivinghoe Beacon on the west. Between the Gault and the Chalk 
Marl there was usually a narrow band of the Upper Greensand, 
but it did not appear to be present here. A bed of greenish sand 
at the base of the Chalk Marl, called the Cambridge Greensand, 
was really the lowest bed of the Chalk, for it contains fossils of the 
Chalk, and not those of the Upper Greensand. 
The Chalk, formerly divided into Upper Chalk or Chalk-with- 
flints, and Lower Chalk or Chalk-without-flints, was now divided 
into Upper, Middle, and Lower Chalk. In adopting this triple 
division the palaeontological zones were recognised, and the con¬ 
tinental classification was conformed to. The Upper Chalk was 
left as it was, and it represents the Senonian ; the Chalk below 
was divided into Middle Chalk, representing the Turonian, and 
Lower Chalk, representing the Cenomanian , and where this last 
division is made there was the greatest palaeontological break in 
the whole of the Chalk. 
Their walk was commenced on the Gault, the Chalk Marl was 
then passed over, and then the whole of the Grey Chalk, the last 
20 or 30 feet of the ascent being up an artificial mound. The 
Totternhoe Stone also was crossed, for it occurs between the Chalk 
Marl and the Grey Chalk. After leaving the Gault, therefore, the 
whole of the Lower Chalk here represented had been traversed; 
and presently would be seen, in the quarries below, a section of the 
Totternhoe Stone and Grey Chalk. This, however, was partly 
white, and here about the upper half was so. The Chalk Marl 
was about 70 or 80 feet thick, the Totternhoe Stone at least 20 feet, 
including when present its middle bed of soft chalk, and the Grey 
Chalk about 80 feet, giving a thickness to the Lower Chalk here 
of about 170 or 180 feet. 
Hear the top of the hill above the quarries there was another 
hard bed called the Melbourn Rock, which forms the base of the 
Middle Chalk. Then, forming a great part of the downs up to from 
about 700 to 720 feet in altitude, was the great mass of Chalk-with- 
few-flints; its flints being chiefly near the top and not occurring in 
layers as in the Upper Chalk. A third hard bed, called the Chalk 
Rock, forms the summit of the Middle Chalk. It contains many 
fossils, the prevailing forms being Gastropoda, which are compara¬ 
tively rare elsewhere in the Chalk. The Melbourn Rock was here 
about 10 feet thick, the Chalk Rock 5 feet or more, and the white 
chalk between these beds from 220 to 230 feet. It had hitherto 
been stated to be 350 feet in thickness in Beds and Bucks,^ but 
* See ‘ The Geology of the Neighbourhood of Cambridge,’ by W. H. Penning 
and A. J. Jukes-Browne, p. 21 (1881). The reference here to “ Whitaker, 1865, 
1872,” Mr. Jukes-Browne states, is to his section in his memoirs “ On the Chalk 
of Buckinghamshire, and on the Totternhoe Stone,” in ‘ Quart. Journ. Geol. Soc.,’ 
vol. xxi, p. 398, and on ‘ The Geology of the London Basin.’ 
