
S04 STRATIGRAPHICAL GEOLOGY. [Book VI. _ 
still retaining the conditions of the Glacial Period, though in 
diminished intensity. The present glaciers of the Alps are no doubt 
the lineal descendants of the vaster sheets that once descended into 
the lowlands on all sides from that central elevated region. And 
even where the ice has long since disappeared, there remain, in the 
living plants and animals of the higher and colder uplands, witnesses 
to the former severity of the climate. As that severity lessened, 
the Arctic vegetation that hitherto had peopled all the lower 
erounds of central and western Europe, was driven up into the hills 
before the advance of plants loving a milder temperature, which had 
doubtless been natives of Europe before the period of great cold, 
and which were now enabled to reoccupy the sites whence they had 
been banished. On the higher mountains, where the climate is still 
not wholly uncongenial for them, colonies of the once general Arctic 
flora still survive. The Arctic animals have also been mostly driven 
away to their northern homes, or have become wholly extinct. 
It has been forcibly pointed out by Mr. Wallace that the present 
mammalian fauna of the globe presents everywhere a striking con- 
trast to the extraordinary variety and great size of the mammals of 
the Tertiary periods. “We live,” he says, “in a zoologically im- 
poverished world, from which all the largest, and fiercest, and strangest 
forms have recently disappeared.” * He connects this remarkable 
reduction with the refrigeration of climate during the Glacial Period. 
The change, to whatever cause it may be assigned, is certainly re- 
markably persistent in the Old World and in the New, and not 
merely in the temperate and northern regions, but even as far south 
as the southern slopes of the Himalaya Mountains. | 
§ 2. Local Development. 
Britain.—Though the generalized succession of phenomena above 
eiven is usually observable, some variety is traceable in the evidence in 
different parts of the British area. In Scotland, where the ground is — 
generally more elevated, and where snow and ice were most abundant, 
the phenomena of glaciation reached their maximum development. In 
the high grounds of England, Wales, and Ireland there was likewise 
extensive accumulation of ice. The ice-worn rocks of the low grounds 
are usually covered by boulder-clay, which in Scotland is interstratified 
with beds of sand, fine clay, and peat, marking interglacial terrestrial 
periods, but has never yielded any marine organisms except near the 
coast, where they are sometimes common, and in one locality in Lanark- 
shire. In England, marine shells, usually fragmentary, occur in the 
boulder-clays both in the eastern and western counties. The ice-sheet 
no doubt passed over some parts of the sea-bottom, and ground up the 
shell-banks that happened to lie in its way, as has happened, for 
example, in Caithness, Holderness, and East Anglia, where the shells 
in the boulder-clay are fragmentary, and sometimes ice-striated. The 
‘« Bridlington Crag” of Yorkshire is regarded by Mr. C. Reid as a large 
' Geographical Distribution of Animals, i. p. 150. 
