90 THE NATURALIST IN NICARAGUA. [Ch. VI. 



few fathoms of the surface. When, however, these 

 deposits were followed downwards, they invariably got 

 poorer, and at one hundred feet from the surface, no 

 very rich ore has been met with. Below that, when 

 the works are prosecuted still deeper, there does not 

 appear to be any further progressive deterioration in 

 the value of the ore, and it varies in yield from two to 

 seven pennyweights of gold per ton, upon which yield 

 further depth does not seem, to have any effect. The cause 

 of these rich deposits near the surface does not appear 

 to me to be that the lodes originally, before they were 

 exposed by denudation, contained more gold in their 

 upper portions than below, but to be the effect of the 

 decomposition, and wearing down of the higher parts, 

 and the concentration of the gold they contained in the 

 lode below that worn away. We have seen that in the 

 decomposed parts of the lode, the gold exists in loose 

 fine grains. During the wet season water percolates freely 

 from the surface down through the lodes, and the gold 

 set free by the decomposition of the ore at the surface 

 must be carried down into it, so that in the course of ages, 

 during the gradual degradation and wearing away of the 

 surface, there has, I believe, been an accumulation of 

 the loose gold in. the upper parts of the lodes from parts 

 that originally stood much higher, and have now been 

 worn away by the action of the elements. 



This accumulation of loose gold near the surface of 

 auriferous veins, set at liberty from its matrix by the 

 decomposition of the ore, and concentrated by degrada- 

 tion, is probably the reason of the great richness of 

 many of what are called the caps of quartz veins ; that 

 is, the parts next the existing surface, and has, also, 



