IN THE NEIGHBOURHOOD OF ALNWICK. 355 
porphyry I have never seen ; greywacke of any size is rarely to 
be met with. 
While, however, it may be concluded that the boulder clay sand 
and gravel beds of Northumberland were not the result of sub- 
aérial glacial action, there are residual phenomena to be accounted 
for. It may be questioned whether even great waves could bear 
onward, over steep vales and high hills, for the distance of above 
100 miles, the large granite blocks found in the formation. 
Granting that this could be effected, is it probable that these blocks 
could travel over so rough a road, and yet retain all their sharp 
corners? Though waves of translation might sweep onward, as has 
been represented, with resistless force, large blocks and immense 
masses of gravel and sand, yet whatever may be their powers of 
transport, they could not produce the bright polishing and the re- 
cular striations of rocks ; for rocks, having their weight much 
lessened when under water, do not supply the pressure necessary 
to bring out a polished surface. No such effect has as yet been 
observed to result from the actions of tides or currents, and the 
effect of a powerful current, on a mass of loose rocks or debris, 
would not be to move the whole in mass, but to break it up and 
to scatter the fragments. 
The agents which seem most likely to have transported the far 
travelled blocks, and to polish and striate rocks, are icebergs. 
Glaciers, itis well known, protrude into the sea in high latitudes, 
both in the northern and southern hemispheres; ceaselessly 
moving onward, they bring down from the higher grounds the 
rocks which have fallen upoh them ; and as large masses of ice are 
from time to time detached, they are floated away by the marine 
currents. Sir John Ross saw one of these stone-laden ice-floes 
in 66° §. lat., on which there were not only stones and mud, but 
also a large basaltic block weighing many tons ; another tabular 
mass of ice laden with rocks, three quarters of a mile in circum- 
ference, and rising 130 feet above the water, was observed in 
58°36 §. lat. These enormous masses move with a force 
which nothing can-resist: even the ice-floes, which are carried 
down the river St. Lawrence, snap with ease the strongest chain 
cable. When, therefore, an iceberg grounds in shallow water, or 
