424 ‘DYNAMICAL GEOLOGY. ——‘[Boox II. | 
noticed the origin and movements of the ice which in cireumpolar 
latitudes covers the sea. This ice is derived from two sources—a, — 
the freezing of the sea itself, and 8, the seaward prolongation of 
land-ice. 
a. Three chief types of sea-ice have been observed. (a.) In the 
Arctic sounds and bays the littoral waters freeze along the shores 
and form a cake of ice which, upborne by the tide and adhering to 
the land, is thickened by successive additions below, as well as by 
snow aboye, until it forms a shelf of ice 120 to 130 feet broad and 
20 to 30 feet high. This shelf, known as the ice-foot, serves as a 
platform on which the abundant débris, loosened by the severe frosts 
of an Arctic winter, gathers at the foot of the cliffs, It is more or 





















































































































































































































































































































Fic. 157 —Disruprep Fior-1cz or Aroric SEAS. 
less completely broken up in summer, but forms again with the early 
frosts of the ensuing autumn. (0.) The surface of the open sea 
likewise freezes over into a continuous solid sheet, which, when 
undisturbed, becomes in the Arctic regions about eight feet thick, but 
which in summer breaks up into separate masses, sometimes of large 
extent, and is apt to be piled up into huge, irregular heaps. This is 
what navigators term floe-ice, and the separate floating cakes are 
known as floes. Ships fixed among these floes have been drifted 
with the ice for hundreds of miles until at last liberated by 
its disruption. (c.) In the Baltic Sea, off the coast of Labrador 
and elsewhere, ice has been observed to form on the sea-bottom. 
SS ee 



