and Motion of Glaciers. 



375 



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sufficient to produce the blue veins, 

 which, it is affirmed, are " an inte- 

 gral part of the inmost structure " 

 of the ice. Saussure was of opinion 

 that the frosts of winter did not 

 penetrate to a greater depth than 

 10 feet, even at the summit of Mont 

 Blanc, and Professor Forbes con- 

 siders this opinion to be a just one. 

 But if so, there would be some diffi- 

 culty in referring to the frosts of 

 winter the blue veins which M. 

 Agassiz observed at a depth of 120 

 feet below the surface of the glacier 

 of the Aar. 



2. It will be remembered that 

 M. Guyot's statement regarding 

 the blue veins is, that he saw the 

 mass of the glacier composed of a 

 multitude of layers of white ice, 

 separated, each from the other, by 

 a plate of transparent ice. The 

 description of Professor Forbes is 

 brieflythis:— "Laminae orthin plates 

 of transparent blue ice, alternate in 

 most parts of every glacier with 

 laminae of ice, not less hard and 

 perfect, but filled with countless 

 air-bubbles which give it a frothy 

 semitransparent look." But there 

 is another form of the blue veins, 

 already referred to, which consists 

 in transparent lenticular masses im- 

 bedded in the general substance of 

 the white ice. Horizontal sections 

 of these transparent lenses were ex- 

 posed upon the surface of the Grin- 

 delwald glacier, and vertical sec- 

 tions of them upon the perpendi- 

 cular sides of the water-courses, and 

 upon the walls of the crevasses. 

 The following measurements, taken 

 on the spot, will give an idea of 

 their varying dimensions. Such 

 masses as these here figured were 

 distributed in considerable numbers 



