7r^ 



542 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



nerative quantity. Most of these detrital deposits, where rich enough 

 to pay for working, have already been washed over, and have become 

 exhausted of their gold. Those which are still the object of exploita- 

 tion on a large scale usually produce diamonds as well as gold ; as, for 

 instance, Australia and California, not to speak of the less important 

 Appalachian gold-fields. It is true that the quantity of diamonds thus 

 produced is very small, so that there would not be the slightest proba- 

 bility that the auriferous gravels could ordinarily be worked with profit 

 for this gem ] although we have reason to believe that by the common 

 methods of gold-washing much the larger proportion of the diamonds 

 would be lost or passed over unseen. Still, we are obliged to admit 

 that the diamond must originally have had its birthplace in the same 

 crystalline rocks from which the gold has been derived, and out of 

 whose detritus both these precious substances are washed. There is no 

 more reason to suppose that the diamonds were formed in the gravel 

 posterior to its abrasion from the solid rock than that the gold was. 



The occurrence of the diamond in Brazil, in the detrital formations, 

 in no respect differs from that indicated for California and Australia, 

 except that these are localities where this gem is unusually abundant, 

 and where the detrital material is more ferruginous than it ordinarily is. 

 The diamonds have been deposited in a modern gravel, made up of the 

 ruins of a much older crystalline formation ; and this gravel has been, 

 in places, reconsolidated into a rock by having become impregnated 

 with a ferruginous solution. Whether the diamonds came originally 

 out of the crystalline metamorphosed sedimentary rocks, or out of the 

 eruptive masses with which these are associated — the eruptive mate- 

 rial ofteuN predominating very largely over the sedimentary : this we 

 know not, as the diamonds are always found loose, in crystals, and sepa- 

 rate from any of their original matrix. 



In fact, it is only within a few years that we knew anything definite 

 about the occurrence of the diamond in its original matrix. The 

 locality we refer to is, of course, South Africa, where this gem occurs, 

 within certain well-defined limits, in much larger quantity than was 

 ever before known. But here the matrix, or enclosing rock, is unques- 

 tionably eruptive. It not only has the mineralogical characters of an 

 eruptive nn^k, but the whole style of its occurrence is such as to stamp 

 it as unmistakably volcanic. Everything about the South African 

 diamond-bearing localities is remarkable and exceptional; and thus Hir 

 the theory of the origin of the diamond has been but little advanced 

 by the observations made in that region. All that can safely be said 



