Cb. VI.] QUARTZ LODES. 87 



nearly vertically, excepting that they are very irregular 

 in thickness, and often branch and send thin offshoots 

 into the enclosing rocks ; they resemble coal seams that 

 haye been turned up on edge, so as to be vertical instead 

 of horizontal. They run for a great distance. Near 

 Santo Domingo they have been traced for two miles in 

 length, and probably they extend much further. They 

 are what are called fissure-veins, owing their origin to 

 cracks or fractures in the rocks that have been filled up 

 with mineral substances through chemical, thermal, 

 aqueous, or Plutonic agencies. In depth, the bottom of 

 fissure- veins has never been reached, and taking into 

 consideration the deep-seated forces required to produce 

 fissures of such great length and regularity, we may 

 safely assume that they run for miles deep into the earth 

 that their extension vertically is as great as it is hori- 

 zontally. The probability that they extend to immense 

 depths is increased when we reflect that mineral veins 

 occur in parallel groups that run with great regularity 

 for hundreds of miles ; and further by the fact that, in all 

 the changes of the earth's surface, by which deep-seated 

 rocks have been brought up and exposed by denudation, 

 no instance is known of the bottom of a fissure- vein 

 having been brought by such movements within the reach 

 of man. 



The gold-mines of Santo Domingo are in veins or lodes 

 of auriferous quartz that run parallel to each other, and 

 are so numerous that across a band more than a mile in 

 width one may be found every fifty yards. All that have 

 been worked vary greatly in thickness : sometimes within 

 a hundred yards a lode will thicken out from one to 

 seventeen feet. Their auriferous contents vary still more 



