10 



THE GEOLOGIST. 



the position of these isolated blocks has become less secure 

 from subsequent and gradual changes in the icy pedestal 

 upon which they rested^ that they are precipitated by the 

 force of gravitation. 



It is from these causes that the glaciers are almost always 

 terminated, and their courses marked by large mounds of 

 fragments of rock, which have received the appellation of 

 moraines, and which exist under determinate circumstances. 

 Mons. Agassiz has classed them under three distinct names, 

 viz. : 



Terminal moraines moraines terminales''— " Gletscher- 

 shutt") when they form the extremity of a glacier. 



Lateral moraines (" moraines laterales'^ — ^" Gandecken^') 

 when they form the lateral boundary of a glacier. 



Medial morabies — C^^moraines medianes" — " GufFerlinien") 

 when they are formed by the junction of those of any two 

 or more vallies. 



These are the most distinct effects produced, and corro- 

 borated by glaciers in existence, as well as by similar traces 

 left by those which have retrograded considerably ; in addi- 

 - tion, the course of a glacier is always marked by numerous 

 characteristic or incidental circumstances, for we find in 

 all situations which are or have been subject to their in- 

 fluence, traces of their action ; such as the polished surface 

 of the rocks, and a striated appearance, produced by the 

 rapid passage over them, of quartzose or gravelly fragments 

 which, as we have seen, occupy the lower portions of all 

 glaciers. These efi'ects are natural, for, considering that the 

 influence of glacial action is manifested most distinctly under 

 circumstances when the opposing force is least, it is clear, 

 that where it is so great that it cannot be permanetnly re- 

 moved by the same cause, some less considerable or re- 

 markable change must take place. 



The " gravel cones,'^ of which mention is made by the 

 various authors alluded to, consist of projections in the 



