366 



GKOUND-ICE AND GLACIERS. 



[Ch. XVI. 



every 



them 



20, and even 30 feet, bringing with it a continual supply of 

 large stones or boulders, and carrying away others ; the 

 greatest number being deposited, according to Lieutenant 

 Bowen, on the edge of deep water. On the island d!, on the 

 left of the accompanying view, a lighthouse is represented, 

 consisting of a square wooden building, which having no 

 other foundation than the boulders, requires to be taken down 

 every winter, and rebuilt on the re-opening of the river. 



These effects of frost, which are so striking on the St. 

 Lawrence above Quebec, are by no means displayed on a 

 smaller scale below that city, where the gulf rises and falls 

 with the tide. On the contrary, it is in the estuary, between 

 the latitudes 47° and 49°, that the greatest quantity of gravel 

 and boulders of large dimensions are carried down annually 

 towards the sea.* Here the frost is so intense, that a dense 

 sheet of ice is formed at low water, which, on the rise of the 

 tide, is lifted up, broken, and thrown in heaps on the ex- 

 tensive shoals which border the estuary. When the tide 

 recedes, this packed ice is exposed to a temperature some- 

 times 30° below zero, which freezes together all the loose 

 pieces of ice, as well as the granitic and other boulders. The 

 whole of these are often swept away by a high tide, or when 



the river is 



m 



snow in spring 



One huge block of granite, 15 feet long by 10 feet both in 



* 



width and height, and estimated to contain 1,500 cubic feet, 

 was conveyed in this manner some distance in the year 1837, 

 its previous position being well known, as up to that time it 



mark 



station. 



Ground-ice. — When a current of cold air passes over 



the 



surface of a lake or stream it abstracts from it a quantity 

 of heat, and the specific gravity of the water being thereby 

 increased, the cooled portion sinks. This circulation may 

 continue until the whole body of fluid has been reduced to 

 the temperature of 40° F., after which, if the cold increase, 

 the vertical movement ceases, the water which is uppermost 

 expands and floats over the heavier fluid below, and when it 





