Erratics of the Alps. 309 



of the most irregular figures, and often three inches or more in 

 length. According to Agassiz, it consists of fragments from half 

 an inch to an inch and a half in breadth, increasing to three inches 

 at the lower end of the glacier. The fissures, says Professor Forbes, 

 admit the free infiltration of water to great depths, and impart to 

 the mass " a certain rude flexibility within narrow limits." As 

 evidence of this flexibility, it may be sufficient to mention two facts. 

 First, the middle of a glacier moves faster than the sides, shewing 

 that the constituent polyhedrons of the ice separate from, and glide 

 over, or pass one another. Secondly, a glacier accommodates itself 

 to the dimensions of its bed ; it contracts its breadth when it has to 

 pass through a narrow gorge, and expands again when the bed widens. 

 Its motions, in short, resemble those of tar or mortar or mud on an 

 inclined plane. 



The extreme facility with which water obeys the law of gravitation 

 is well known. The " rapid" Rhone, according to M. de Beaumont, 

 has a mean fall of only V 54" or about 3 feet per mile, from Lyons 

 to Aries. Nay, there are portions of the Rhine and the Seine, he 

 says, where the declivity is so small as 8, or even 4 seconds — that 

 is, a fall of one foot in 25,000 or 50,000. Sir Charles Lyell states 

 that the surface of the Mississippi, at its junction with the Ohio, has 

 an elevation of no more than 200 feet above the Gulf of Mexico. 

 The round number of 200 may raise a doubt whether it is the result 

 of careful measurement; but if so, the fall must be only about 

 2 inches per mile ; for the length of the river below the junction, 

 including all its sinuosities, is estimated in "Darby's" Louisiana at 

 1175 miles. The component parts of a glacier, however, want the 

 mobility of the molecules of water, and the motion of the former is 

 better illustrated by the troughs filled with plaster of Paris in Pro- 

 fessor Forbes' s ingenious experiment. We have another illustration 

 in the flowing of a lava current, which, like the glacier itself, has the 

 advantage of being on a grand scale. M. de Beaumont, in his valu- 

 able Memoir on Mount Etna, gives a table of the declivities or slopes 

 of a great number of currents in active or extinct volcanoes. In 

 22 of these the slope was under 3 degrees; in 10 it was under 2 

 degrees ; in 5 under 1 degree (that is, less than 1 foot in 57), and 

 in the vast currents which flowed from the Icelandic volcano of 

 Skapta Jokul in the terrible eruption of 1783, to an extent of 50 

 miles, the slope was only 30 minutes, or a fall of 1 foot in 114. 

 This was the mean slope, and at some parts the actual fall must 

 have been still less. Now, the fluidity of lava is much less perfect 

 than that of water, even at the moment when it issues from the 

 crater, and when passing through the pasty condition before it be- 

 comes solid, as in the lower part of a coulee, may fitly bear a com- 

 parison with glacier ice. In its most liquid state, a large stone 

 thrown upon it floats on its surface, as a loaf floats on honey, which 

 it resembles in consistency. A coulee, eight feet broad, which I saw 



