190 
ELEMENTS OF GEOLOGY. * 
north. Fifteen species are British and Arctic, a very few 
belong to those species which range south of our British 
seas. Five species or well-marked varieties are not known 
living, namely, the variety of Astarte borealis (called A, 
Withami ); A, mutabilis ; the sinistral form of Tritonium 
carmatum, CarcUta analis^ and Tellina obliqua^ Fig. 120, p. 
194. Mr. Searles Wood also inclines to consider Nucula 
Cobboldice^ Fig. 119, p. 194, now absent from the European 
seas and the Atlantic, as specifically distinct from a closely- 
allied shell now living in the seas surrounding Vancouver’s 
Island, which some conchologists regard as a variety. TeU 
Una obliqua also approaches very near to a shell now living 
in Japan. 
Glacial Drift of Ireland. —Marine drift containing the last- 
mentioned Nucnla and other glacial shells reaches a height 
of from 1000 to 1200 feet in the county of Wexford, south 
of Dublin. More than eighty species have already been ob¬ 
tained from this formation, of which two, Conovulus pyra- 
midalis and N'assa monensis^ are not known as living; while 
Turritella incrassata and Cyprma lucida no longer inhabit 
the British seas, but occur in the Mediterranean. The great 
elevation of these shells, and the still greater height to 
which the surface of the rocks in the mountainous regions 
of Ireland have been smoothed and striated by ice-action, 
has led geologists to the opinion that that island, like the 
greater part of England and Scotland, after having been 
united with the continent of Europe, from whence it re¬ 
ceived the plants and animals now inhabiting it, was in 
great part submerged. The conversion of this and other 
parts of Great Britain into an archipelago was followed by 
a re-elevation of land and a second continental period. Af¬ 
ter all these changes the final separation of Ireland from 
Great Britain took place, and this event has been supposed 
to have preceded the opening of the straits of Dover.^ 
Drift of Norfolk Cliifs. —There are deposits of boulder clay 
and till in the Norfolk cliffs principally made up of the waste 
of white chalk- and 
flints which, in the 
opinion of Mr. Searles 
Wood, jun., and oth¬ 
ers, are older than the 
Bridlington drift, and 
contain a larger pro¬ 
portion of shells com¬ 
mon to the Norwich and Red Crag, including a certain num- 
* See Antiquity of Man, chap. xiv. 
