OUTLINES OF GEOLOGY. 



11 



portions of a continent by supposing lines to be drawn through 

 the sources of the several streams. 



23. The valley of a stream is sometimes divided into two 

 or more separate basins by ridges of mountains, through 

 ravines in which the river forces its way. Of this we have 

 an instance in the Hudson, and one still more remarkable in 

 the Susquehannah. The main branch of the latter river 

 occupies a shallow basin, in a country whose general slope 

 is directed towards the lakes ; and in making its way to the 

 sea it passes through five distinct ranges of mountains. 



24. Mountains may be either isolated, combined in groups, 

 or united in ridges. Groups of mountains are generally ar- 

 ranged around a central eminence, which has a greater 

 height than any which surround it. In some cases, the 

 whole group is considered and named as if it were a single 

 isolated mountain. Of this we have an instance in Mtna, 

 which is composed of a number of separate mountains, lying 

 upon the slopes of a greater central peak. 



25. Chains of mountains have sometimes a constant direc- 

 tion throughout their whole extent. Such is the case with 

 the Pyrenees : at other times they suddenly change their 

 direction, or follow irregular lines. In spite of any irregu- 

 larity, it may be generally remarked, that principal chains 

 of mountains have a direction analogous to that of the land 

 in which they are situated. 



20. The breadth of chains of mountains is extremely 

 variable, and they sometimes exhibit a continuous crest, va- 

 rying but little in height ; in other cases and more usually, 

 the crest is deeply serrated, so that the chain may be consi- 

 dered as made up of a number of detached mountains. The 

 highest points of a chain are called peaks. 



27. The direction of a chain of mountains is usually 

 pointed out by imaginary lines joining the peaks. This is, 



