402 DYNAMICAL GEOLOGY. —_—[Boox IIL. - 
Z ; 2 a 
feet thick. A vast amount of anchor-ice is likewise formed on the ~ 
bottoms of the rivers and rises to the surface. In several ways 
geological changes are thus effected. Mud, gravel, and boulders, 
encased in the anchor-ice or pushed along by it on the bottom, are 
moved from their position. This ice, formed in considerable quantity 
in the rapids of the Canadian rivers, is carried down stream and 
accumulates against the bars and banks or is pushed over upon the 
surface of the upper ice. By its accumulation a temporary barrier 
is formed, the bursting of which causes destructive floods. When 
the ice breaks up in early summer, cakes of it which have formed along 
shore and have enclosed beach-pebbles and boulders, float off so as 
either to drop these in deeper water or to strand them on some other — 
part of the shore. This kind of transport takes place on a great 
seale on the St. Lawrence. ‘The islets of boulder clay and solid rock 
are fringed with blocks which have been stranded by ice and which 
are ready to be again enclosed, and floated off further down stream, 
Should a gale arise during the breaking up of the frost, vast 
piles of ice, with mingled gravel and boulders, may be driven ashore 
and pushed up the beach ; even blocks of stones of considerable size, 
are sometimes forced to a height of several yards, tearing up the — 
soil on their way, and helping to form a bank above the water level. — 
In the same river great destruction of banks has been caused by 
rafts of ice, and particularly of anchor-ice, Crab Island, for example, 
which was about an acre and a half in extent at the beginning of this 
century, has entirely disappeared, its place being indicated merely 
by a strong ripple of the water, which is every year getting deeper 
over the sitet Other islands have also been destroyed. Great 
damage is frequently done to quays and bridges in the same region 
by masses of river-ice driven against them on the arrival of spring. 
Reference has already been made to the increased power of transport 
and erosion acquired by rivers liable to be frozen over, and especially 
when their ice is broken up in the higher parts of their courses, 
before it gives way in the lower (p. 368). : 
Hail, the formation of which is not yet well understood, falls 
chiefly in summer and during thunderstorms. When the pellets of 
ice are frozen together so as toreach the ground in lumps as large as. 
a pigeon’s egg, or larger, great damage is often done to cattle, flying 
birds, and vegetation. Trees have their leaves and fruit torn off, and 
farm crops are beaten down. 
Snow.—In those parts of the earth’s surface where, either 
from geographical position or from elevation into the upper cold 
regions of the atmosphere, the mean annual temperature is below the 
freezing point, the condensed moisture falls chiefly as snow, and 
remains in great measure unmelted throughout the year. <A line 
termed the snow-line can be traced, below which the snow disappears 
in summer, but above which it continues to cover the whole or great 
part of the surface. The snow-line comes down to the sea within the 
’ Bleasdell, Q, J. Geol. Soc, xxvi. p. 669; xxviii. p. 292, 

