8G THE NATURALIST IN NICARAGUA. [Ch. VI. 



the commencement of new valleys, and the widening and 

 lengthening of others, caused not only by the gradual 

 denudation of the surface, but by landslips, some of 

 which occur every wet season. 



The rocks of the district are dolerytes, with bands and 

 protrusions of hard greenstones. The decomposition of 

 the dolerytes is very great, and extends from the tops of 

 the hills to a depth, as proved in the mines, of at least 

 two hundred feet. Next the surface they were often as 

 soft as alluvial clay, and might be cut with a spade. This 

 decomposition of the rocks near the surface prevails in 

 many parts of tropical America, and is principally, if not 

 always, confined to the forest regions. It has been 

 ascribed, and probably with reason, to the percolation 

 through the rocks of rain -water charged with a little acid 

 from the decomposing vegetation. If this be so, the 

 great depth to which it has reached tells of the immense 

 antiquity of the forests. 



Gold-mining at Santo Domingo is confined almost 

 entirely to auriferous quartz lodes, no alluvial deposits 

 having been found that will pay for working. The lodes 

 run east and west, and are nearly perpendicular, some- 

 times dipping a little to the north, sometimes a little to 

 the south, and near the surface, generally turning over 

 towards the face of the hill through which they cut. 

 The trend of the main ranges, also nearly east and west, 

 is probably due to the direction of the outcrops of the 

 lodes which have resisted the action of the elements 

 better than the soft dolerytes. The quartz veins now 

 form the crests of many of the ranges, but are everywhere 

 cut through by the lateral valleys. The beds of doleryte 

 lie at low angles, through which the quartz veins cut 



