890 STRATIGRAPHICAL GEOLOGY. [Boox VI]. 
source. ‘Thus in the stony clay and gravel of the plains of northern 
Germany and Holland, besides the abundant locally-derived detritus, 
fragments occur which have had an unquestionably northern origin. 
Some of the rocks of Scandinavia, Finland, and the Upper Baltic are 
of so distinctive a kind that they can be recognized in small pieces. 
Thus the peculiar syenite of Laurwig in the south of Norway has 
been recognized abundantly in the drift of Denmark ; it occurs also in 
that of Hamburg, and has been detected even in the boulder-clay of 
the Holderness cliffsin Yorkshire. The well-known rhombenporphyr 
of southern Norway has likewise been recognized at Holderness. 
Fragments of the Silurian rocks from Gothland, or from the Russian 
islands Dago or Oesel, have been met with as far as the north of 
Holland. Pieces of granite, gneiss, various schists, porphyries, and 
other rocks, probably from the north of Europe, occur in the till of 
Norfolk.’ These transported fragments are an impressive testimony 
to the movements of the northern ice. No Scandinavian blocks have 
been met with in Scotland, for the ice in that country was massive 
_ enough to move out inio the basin of the North Sea (then doubtless 
in great part usurped by glaciers) until it met that which was 
streaming down from Scandinavia and thus kept it from bringing its 
freight of rock debris. But the Norwegian ice-sheet, which crept 
southwards across Denmark, once extended across the North Sea to 
the Yorkshire and Norfolk coasts, unless we suppose that the Scandi- 
navian stones of Holderness and Cromer were carried on floating ice. 
The stones in the boulder-clay have a characteristic form and 
surface. They are usually oblong, have one or more flat sides or 
“soles,” are smoothed or polished, and have their edges worn round 
(Fig. 154). Where they consist of a fine-grained enduring rock, they 
are almost invariably found to be striated, the strie running on the 
whole with the long axis of the stone, though one set of scratches may 
be seen crossing and partially effacing another, which would neces- 
sarily happen as the stones shifted their position under theice. ‘These 
markings are precisely similar to those on the solid rocks underneath 
the bouider-clay, and have manifestly been produced in the same 
way by the friction of stones and grains of sand as the whole mass of 
debris was being steadily pushed on in one general direction. 
Interglacial Beds.—The boulder-clay is not one uniform 
mass of material. In a limited section, indeed, it usually appears as 
an unstratified mass of stiff stony clay. But it is found on further 
examination to be split up with various inconstant and local inter- 
stratifications, and in fact to consist of a group of deposits of different 
ages and formed under very various conditions. Beds of sand, gravel, 
fine clay, and peaty layers on different platforms in the boulder-clay, 
bear witness to intervals when the ice retired from the land, which, so 
! These erratics from their petrographical characters appear to me to be certainly 
not from Scotland. Had that been their source they could not have failed to be accom- 
panied by abundant fragments of the rocks of the south of Scotland, which are con- 
spicuously absent, 
