GAULT—BLACKDOWN BEDS. 
301 
usually about 100 feet Fig. 272 . 
thick in the S.E. of 
England, is provincially 
termed Gault. It con¬ 
sists of a dark blue marl, 
sometimes intermixed 
with green sand. Many 
peculiar forms of ceph¬ 
alopoda, such as the 
Hamite (Fig. 272), and 
Scaphite, with other 
fossils, characterize this 
formation, which, small 
as is its thickness, can be traced by its organic remains to 
distant parts of Europe, as, for example, to the Alps. 
Twenty-one species of British Ammonites are recorded as 
found in the Gault, of which only eight are peculiar to it, ten 
being common to the overlying Chloritic series. 
Connection between Upper and Lower Cretaceous Strata — 
Blackdown Beds. —The break between the Upper and Lower 
Cretaceous formations will be appreciated when it is stated 
that, although the Neocomian contains 31 species of Am¬ 
monite, and the Gault, as we have seen, 21, there are only 3 
of those common to both divisions. Nevertheless, we may 
expect the discovery in England, and still more when we 
extend our survey to the Continent, of beds of passage inter¬ 
mediate between the Upper and Lower Cretaceous. Even 
now the Blackdown beds in Devonshire, which rest imme¬ 
diately on Triassic strata, and which evidently belong to 
some part of the Cretaceous series, have been referred by 
some geologists to the Upper group, by others to the Lower 
or Neocomian. They resemble the Folkestone beds of the 
latter series in mineral character, and 59 out of 156 of their 
fossil mollusca are common to them; but they have also 16 
species common to the Gault, and 20 to the overlying Chlo¬ 
ritic series; and what is very important, out of seven Am¬ 
monites six are found also in the Gault and Chloritic series, 
only one being peculiar to the Blackdown beds. 
Professor Ramsay has remarked that there is a strati- 
graphical break; for in Kent, Surrey, and Sussex, at those 
few points where there are exposures of junctions of the 
Gault and Neocomian, the surface of the latter has been 
much eroded or denuded, while to the westward of the great 
chalk escarpment the unconformability of the two groups 
is equally striking. At Blackdown this unconformability is 
still more marked, for though distant only 100 miles from 
