The Glacial Theory of Prof. Agassiz. 353 



When a conapound glacier is long, the different moraines, lateral and 

 medial, sometimes become blended in their progress downwards, and 

 spread out into a broad sheet ; and if the ice were to disappear, we 

 should find the whole bottom of the valley at this part covered with a 

 confused assemblage of fragments of rock. This is also a deposit oc- 

 casionally met with in Scotland. 



The materials of moraines are not stratified, but huddled together in, 

 confusion. The fragments are generally somewhat rounded by mutual 

 attrition, but some are angular. They may be distinguished from the 

 banks of gravel formed at the margin of lakes by their internal struc- 

 ture, by the difference of level between their distant parts, and also by 

 their form. 



We are not quite sure of the precise shape of terminal moraines, 

 but the terms employed by Agassiz {digues ou remparts) lead us to 

 suppose that they form long mounds with rounded sides. Like the 

 others, they are not stratified internally ; but, from the manner of their 

 formation, they contain, more finely triturated matter, namely, clay, 

 sand, and small gravel. Agassiz seldom gives precise measurements ; 

 but he mentions one terminal moraine, (that of Viesch,) which is thirty 

 feet high, and much more in breadth. Glaciers sometimes advance for 

 a term of years, and then retreat for another term. When a glacier 

 is retreating, it forms a new terminal moraine every year, and when it 

 again advances, it pushes the more recent ones before it till the whole 

 are blended into one mass. Now, if the disappearance of the glaciers 

 took place gradually, as it seems most reasonable to suppose, we ought 

 to find in the lower end of some of our valleys a series of little trans- 

 verse mounds, like a:, i/, in figure 5, below. 



Lateral moraines increase in size towards the lower end of the val- 

 ley, and for an obvious reason : The fragments which fall at the head 

 of the valley are slowly carried downwards by the glacier in its course, 

 and they are joined in their progress by those which fall from the rocks 

 in the lower part of the valley. Blocks which fall into the neve or 

 granular snow high up, sink into it and disappear for a time ; but it is 

 curious, that except those which tumble into crevasses and reach the 

 bottom, they all afterwards rise to the surface. Agassiz thinks, that 

 the internal dilatation which makes the glacier travel downwards, also 

 operates upwards, and carries all included masses to the surface. It is 

 certain that an enclosed boulder is never seen in the terminal section of 

 a glacier, Avhere the composition of the mass can be best observed. In 

 consequence also of the sides of the glacier travelling faster than 

 the middle, and of its breadth generally diminishing towards its lower 

 end, it very often happens that the blocks of medial moraines find their 

 way to the sides and join the lateral ones. 

 Vol. xLii, No. 2.— Jan.-Maich, 1842. 45 



