822 “STRATIGRAPHICAL GEOLOGY. [Boox VI, _ 

Following the modern classification, we find that the old subdivision 
of “Chalk without flints” agrees on the whole with the Turonian 
section of the system. This division, as above remarked, appears in 
some places to he unconformably upon the members below it, from which 
it is further separated by a marked zoological break. Nearly all the 
Cenomanian species now disappear save two or three cosmopolitan forms. 
The echinoderms and brachiopods are entirely replaced by new species." 
Not only is the base of the Turonian group defined by a stratigraphical 
hiatus, but its summit is marked by the Nodular Chalk of Dover and 
the hard Chalk-rock, which appear to indicate another stratigraphical 
break in what was ‘formerly believed to be an uninterrupted deposit 
of chalk. The three Turonian paleontological zones, so well established 
in France, are also traceable in England. As exposed in the splendid 
Kent cliffs, the base of the English beds is formed by a well-marked 
band (82 feet) of hard gritty chalk, made up of fragments of Inocerami 
and other organisms. Fossils are here scarce; they include Inoceramus 
labiaius (which begins here), Rhynchonella Cuvieri, Echinoconus subrotundus, 
Cardiaster pygmeus. Above this basement bed lies the massive chalk 
without flints, full of fragments of Inoceramus labiatus, with I. Cuvieri, 
Terebratula semiglobosa, Terebratulina gracilis, Echinoconus subrotundus, &c. 
The lower 70 feet or so include the zone of Inoceramus labiatus, the next 90 
or 100 feet that of Terebratulina gracilis, and the upper 50 or 60 feet, con- 
taining layers of black flints, that of Holaster planus. At the top comes 
the remarkably constant band of hard cream-coloured limestone known. 
as the “ Chalk-rock,” varying from a few inches to 10 feet in thickness. 
Its upper surface is generally well defined, sometimes even snp er rLaD of 
having beer eroded, but it shades down into the lower chalk,’ 
Senonian ( Upper Chalk with jflints)—This massive formation is 
composed of white pulverulent and usually tolerably pure chalk, with — 
scattered flints, which, being arranged in the lines of deposit, serve to 
indicate the otherwise indistinct’ stratification of the mass. It has 
been generally regarded by English geologists as a single formation, 
with great uniformity of lithological characters and fossil contents. Mr. 
Whitaker, however, has shown that distinct lithological platforms occur 
in it, and more recent researches, especially by MM. Hébert and Barrois, 
have brought to hght the same zones that occur in the Paris basin. Of 
these the “lowest, or that of the Micrasters (Broadstairs and St. Mar- 
garet’s chalk), is most widely spread, the others having suffered most 
from denudation. It is well exposed along the cliffs of Kent at Dover, 
and also in the Isle of Thanet.. At Margate its thickness has been 
ascertained by boring to be 265 feet. It “contains two zones, in the 
lower of which the characteristic urchin is Micraster cor-testudinarium, 
while in the upper it is M. cor-anguinum. Near the top of the Micraster 
group of beds in the Isle of Thanet, lies a remarkable seam of flint about 
three or four inches thick, forming a nearly continuous floor, which 
has been traced southwards at the “top of the cliffs between Deal and 
Dover. Again, on the coast of Sussex, the same horizon in the chalk is 
defined by a corresponding band of massive flattened flints. The traces of 
emersion and erosion observed by M. Hébert in the Paris chalk are 
' Jukes-Browne, Greol. Mag. 1880, p. 250. 
* Whitaker, Mem, Geol. Surv. iy. p. 46. Jukes-Browne, Geol. Mag. 1880, p, 254. A 
sinifiax band occurs in Normandy, 
