Ch. II.] 
STRATA AT SUCCESSIVE PERIODS. 
19 
been pointed out by the older Italian writers, and some correct 
ideas, as we have seen^ had been entertained respecting their 
recent origin, as compared to the inclined secondary rocks on 
which they rested*. But accurate .data were now for the first 
time collected, for instituting a comparison between them and 
other members of the great European series of tertiary for- 
mations. 
Brocchi came to the conclusion that nearly one-half of 
several hundred species of fossil shells procured by him from 
these Subapennine beds were identical with those now living 
in existing seas, an observation which did not hold true in 
respect to the organic remains of the Paris basin. It might 
have been supposed that this important point of discrepancy 
would at once have engendered great doubt as to the identity, 
in age, of any part of the Subapennine beds to any one mem- 
ber of the Parisian series ; but, for reasons above alluded to, 
this objection was not thought of much weight, and it was 
supposed that a group of strata, called c the upper marine for- 
mation,' in the basin of the Seine, might be represented by all 
the Subapennine clays and yellow sand. 
English Crag. — Several years before, an English naturalist, 
Mr. Parkinson, had observed, that certain shelly strata, in 
SmTolk, which overlaid the blue clay of London, contained 
distinct fossil species of testacea, and that a considerable por- 
tion of these might be identified with species now inhabiting 
the neighbouring sea-f. These overlying beds, which were 
provincially termed ' Crag,' were of small thickness, and were 
not regarded as of much geological importance. But when 
duly considered, they presented a fact worthy of great atten- 
tion, viz., the superposition of a tertiary group, inclosing, like 
the Subapennine beds, a great intermixture of recent species 
of shells, upon beds wherein a very few remains of recent or 
living species were entombed. 
Mr. Conybeare, in his excellent classification of the English 
* See vol. i. p. 51, for opinions of Odoardi, in 1761. 
f Geol. Trans., vol. i. p. 324. 1811. 
C 2 
