330 
ELEMENTS OF GEOLOGY. 
The Sables de Bracheux, enumerated in the Tertiary divis¬ 
ion of the table, supposed by Mr. Prestwich to be some¬ 
what newer than the Thanet Sands, and by M. Hebert to be 
of about that age, have yielded at La Fere the Arctocyon 
{Palceocyo7i) primcevus^ the oldest known tertiary mammal. 
It is worthy of notice, that in the Hastings Sands there 
are certain layers of clay and sandstone in which numerous 
foot-prints of quadrupeds have been found by Mr. Beckles, 
and traced by him in the same set of rocks through Sussex 
and the Isle of Wight. They appear to belong to three or 
four species of reptiles, and no one of them to any warm¬ 
blooded quadruped. They ought, therefore, to serve as a 
warning to us, when we fail in like manner to detect mam¬ 
malian foot-prints in older rocks (such as the New Red Sand¬ 
stone), to refrain from inferring that quadrupeds, other than 
reptilian, did not exist or pre-exist. 
But the most instructive lesson read to us by the Purbeck 
strata consists in this: They are all, with the exception of 
a few intercalated brackish and marine layers, of fresh-wa¬ 
ter origin; they are 160 feet in thickness, have been well 
searched by skillful, collectors, and by the late Edward 
Forbes in particular, who studied them for months consecu¬ 
tively. They have been numbered, and the contents of each 
stratum recorded separately, by the officers of the Geologic¬ 
al Survey of Great Britain. They have been divided into 
three distinct groups by Forbes, each characterized by the 
same genera of pulmoniferous molltisca and cyprides, these 
genera being represented in each group by different species; 
they have yielded insects of many orders, and the fruits of 
several plants; and lastly, they contain “dirt-beds,” or old 
terrestrial surfaces and vegetable soils at different levels, in 
some of which erect trunks and stumps of cycads and conifers, 
with their roots still attached to them, are preserved. Yet 
when the geologist inquires if any land-animals of a higher 
grade than reptiles lived during any one of these three peri¬ 
ods, the rocks are all silent, save one thin layer a few inches 
in thickness; and this single page of the earth’s history has 
suddenly revealed to us in a few weeks the memorials of so 
many species of fossil mammalia, that they already outnum¬ 
ber those of many a subdivision of the tertiary series, and 
far surpass those of all the other secondary rocks put to¬ 
gether ! 
Lower Parbeclc .—Beneath the thin marine band mention¬ 
ed at p. 324 as the base of the Middle Purbeck, some purely 
fresh-water marls occur, containing species of Cypris (Fig. 
307 a, c), Valvata^ and Limnma^ different from those of the 
