378 Elements of Mineralogy. 



and other veftiges of organic fubftances, they 



are always ftratified. 



The principal granitic ridges are flanked 

 by argillaceous, as thefe are by alluvial moun- 

 tains. 



Of the height of Mountains. 



There is no circumftance relating to moun- 

 tains, that excites the furprife, or interefts the 

 curiofity of mankind to a greater degree than 

 their height ; hence many methods have 

 been devifed to determine it. The trigono- 

 metrical is the moft antient, and in many 

 cafes exadt, but it is often imperfect, and in 

 fome cafes impracticable ; the barometrical 

 alfo remained for a long time imperfeft, until 

 the celebrated Mr. De Luc, with indefatiga- 

 ble induftry, and uncommon fagacity, brought 

 it to a degree of accuracy, " fcarce to be ex- 

 peted. Some improvements have lately 

 been made in it by Sir George Shuckburgh^ 

 and the calculation has been rendered ftill 

 more fimple by Mr. Magellan. 



In this method the elevation of mountains 

 is determined by the different heights of 2 

 columns of mercury, one at the top and the 

 other at the bottom of the mountain, the de- 

 gree 



