146 GEOLOGY. 
of the globe. Hence, valleys thus excavated are termed valleys of erosion, 
from being ground out by the powerful action of these mighty agents. q 
This being oe to be the case even during the human period, we are 
at no loss to account for the great denndaiiion. everywhere seen ; and for 
the immense accumulations of sedimentary matter that form so pes of 
the solid crust of our globe. 
Transporting pe ae have finally to account for the deposition 
of strata in one part of the country, the materials for which have been 
obtained at great distances ; and for the transport of immense boulders 
hundreds of miles from their original seats, as exhibited in all parts of 
the globe. ; 
1. Aqueous Agency.—The most obvious agents of transport are rivers, 
that bear down from every part of their courses the débris deposited at 
their mouths. Their power of carrying masses of the heaviest materials 
is immense, as may be seen after a flood in the smallest streams in our 
neighbourhood. Waves have also a wonderful power of removing and 
carrying to a distance the blocks on which they daily dash. But the 
currents that flow through the ocean, which are but mighty ocean 
rivers, have the greatest influence in this respect. By their means, 
materials of all kinds, organic and inorganic, are conveyed to incredible 
distances. The Gulf-stream, for instance, conveys substances from the 
South African coasts to those of Norway and the far North. 
2. Ice Ageney.—But the transporting influence of these currents in 
bearing rock-masses is greatest when icebergs are carried on their surface. 
These huge frost-mountains have embedded in their mass the largest 
blocks, which are gradually dropped over wide areas as the ice slowly 
melts away. The size and number of some of these transported rocks are 
often almost incredible. Every country exhibits such travelled rocks, 
which are called erratic boulders; and our own little island presents 
. no mean examples of such ice-borne masses. 
But ice also acts as a transporter in the form of glaciers—those great” 
ice-rivers that fill the upland valleys of the Alps, Himalayas, and 
such mountain systems. In front of every glacier, along its sides, and on 
its surface, are great collections of rocky fragments of every size, borne 
down by the ice-stream, and left as evidences of its existence when the 
glacier has melted away. Such collections of rocks are called moraznes, 
from their mural or wall-like aspect as seen running across a valley. The 
distance to which such blocks are borne is astonishing, and depends on the 
size of the glacier. Evidences of extinct glaciers are seen in most countries, 
and. we may trace their remains in our own island, where now not a 
particle of glacier ice can exist. 
1 From Latin e, out, away, and rodo, resum, to gnaw. 

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