266 



ON THE HYDROLOGY OF THE BASIN 



peculiar haze, much resembling our season of early October, in the southern counties of 

 England, and is called, in the transatlantic country, the " Indian Summer." This charming 

 period is suddenly terminated by cold, clear weather, and by about the 25th of November, 

 the still waters become frozen on the surface. Generally by the 5th of the following 

 month, there; is a fall of more or less snow, which, while covering with a protective mantle, 

 and retarding the cooling of the earth, aids the formation of the " ground ice," or " anchor 

 ice." in the streams. 



The author, having attentively watched the formation of this description of ice, recog- 

 nizes it as the precise repetition of the same phenomenon in the Thames and Kennet, of 

 England. 



The results of his observations in England, on the rivers named, have been supported 

 by similar observations in American rivers, and serve to show that the primitive crystals 

 of ice formed first in contact with the cold air, at the surface of the streams, or derived 

 from snow falling thereon, are carried down, by counter-currents and eddies, below the 

 surface, where they become fastened to tin- best conducting media. Occasionally portions 

 of the agglutinated mass are broken off' by the force of the current, or by other causes, — 

 among which may be, and probably is, the change in the atmospheric pressure, and in the 

 temperature of the stream, and consequent change in specific gravity, which would induce 

 the rising of the mass to the surface; and in the large American rivers this soon takes 

 effect, and the whole becomes frozen into a solid mass of field or surface-ice. Much of 

 the ice, however, is for the time held to the bottom, by contact with stones or a rocky bed, 

 and is then known as anchor-ice. Much travels, rolling slowly along on the bed, loaded 

 with gravel or sand, which increasing the specific gravity of the mass, prevents its rising 

 to the surface. 



Soon the small bays and indentations of the banks fill up, and shallows become more 

 effectual barriers to the stream, producing comparatively still waters, which, under the 

 influence of the all-pervading cold, with the increased falls of snow, augment the thickness 

 and strength of the floating masses, Then begins an actual blockade, causing an increase 

 in the height of the river, which relieves, for the moment, some of the ice and the pent up 

 water, which flows towards the next retarding point. 



In the large rivers this is particularly interesting, and some of its features are really 

 imposing. At Montreal, for instance, the movements are possessed of a certain degree of 

 grandeur. Here the current, before described, flowing between the Northwest Bank and 

 St. Helen's Island, although the narrowest channel, soon becomes the master current ; for 

 that which flows through the wider and shallow space, on the south side of the island, 

 although really in the most direct course to the lower reaches of the river, soon becomes 

 almost blocked up. The great discharge of the field-ice, broken up first in the Rapids of 



