368 INCLINATION OF THE STRATA. 



In taking a general view of the geological constitution 

 of a chain of mountains, we may distinguish five elements of 

 direction too often confounded in works of geognosy and 

 physical geography. These elements are : 



1. The longitudinal axis of the whole chain. 



2. The line that divides the waters (divortia aquarum). 



3. The line of ridges or elevation passing along the maxima 

 of height. 



4. The line that separates two contiguous formations into 

 horizontal sections. 



5. The line that follows the fissures of stratification. 



This distinction is the more necessary, there existing pro- 

 bably no chain on the globe that furnishes a perfect parallel- 

 ism of all these directing lines. In the Pyrenees, for instance, 

 1, 2, 3, do not coincide, but 4 and 5 (that is, the different 

 formations which come to light successively, and the direc- 

 tion of the strata) are obviously parallel to 1, or to the 

 direction of the whole chain. We find so often in the most 

 distant parts of the globe, a perfect parallelism between 1 

 and 5, that it may be supposed that the causes which deter- 

 mine the direction of the axis (the angle under which that 

 axis cuts the meridian), are generally linked with causes that 

 determine the direction and inclination of the strata. This 

 direction of the strata is independent of the line of the 

 formations, or their visible limits at the surface of the soil ; 

 the lines 4 and 5 sometimes cross each other, even when one 

 of them coincides with 1, or with the direction of the longi- 

 tudinal axis of the whole chain. The relief of. a country 

 cannot be precisely explained on a map, nor can the most 

 erroneous opinions on the locality and superposition of the 

 strata be avoided, if we do not apprehended with clearness 

 the relation of the directing lines just mentioned. 



In that part of South America to which this memoir prin- 

 cipally relates, and which is bounded by the Amazon on 

 the south, and on the west by the meridian of the Snowy 

 Mountains (Sierra Nevada) of Merida, the different bands or 

 zones of formations (4) are sensibly parallel withthe longitu- 

 dinal axis (1) of the chains of mountains, basins, or interposed 

 plains. It may be said in general that the granitic zone 

 (including under that denomination the rocks of granite, 

 gneiss, and mica-slate) follows the direction of the Cordillera 



