12 



OUTLINES OF GEOLOGY. 



in many cases, a rhumb-line, or directed constantly to the 

 same point of the compass. 



28. What may appear from a low position as a chain of 

 mountains, is often no more than the edge of a table land. 

 There is a famous instance of this in the line intended as a 

 division between the territories of the United States and those 

 of Great Britain. In sailing up the St. Lawrence, the southern 

 shore presents the aspect of a continuous chain of mountains, 

 while, in approaching that river from Maine or New Bruns- 

 wick, no mountains are to be seen, in consequence of the 

 country being an elevated plain, of which the heights seen 

 from the river are the northern and western slope. 



29. The lowest points in a serrated chain of mountains 

 are called passes or gaps. These are the summit levels of 

 valleys which traverse the chain. The slope of a chain is 

 usually estimated as formed on each side of the line which 

 marks its direction by passing through the peaks. A better 

 mode of defining the slope is, by a line passing through the 

 summits of the several gaps. By this method the slopes of 

 the chain are made to correspond with the valleys or basins 

 of the streams which take their rise in them. 



It rarely happens that the opposite slopes of mountains 

 exhibit any approach to uniformity, being, in many cases, 

 abrupt on one side and gentle on the other. In many 

 instances the slopes are composed of a number of transverse 

 ridges separated by valleys. In some cases the slopes are 

 formed by a succession of steps, each having the character 

 of a plain. Chains of mountains are sometimes single ; at 

 other times they are composed of several parallel ridges. The 

 valleys included within systems of mountains may therefore 

 be either transverse or parallel to the general direction of the 

 ridges. 



30. The heights of many mountains have been measured. 



