268 ANNUAL, KEPOKT SMITHSONIAN INSTITUTION, 1923 



guished. These are the true kimberlite tuff or kimberlite breccia, injection 

 breccia, and decomposed kimberlite. * * * As greater depths are attained 

 in the mines the products we have hitherto been dealing with, as one would 

 naturally expect, are replaced in increasing measure by "hardebank," or kim- 

 berlite, the parent rock, to the trituration and decomposition of which they 

 owe their origin. 8 



Wagner divides the diamond-bearing rocks of South Africa into 

 two types, both of which occur as intrusives and as volcanic breccias. 

 One of these is a "basaltic kimberlite," rich in olivine and poor in 

 mica, and the other is a " mica peridotite." Both types are diamond- 

 bearing in South Africa, but the first has produced most of the 

 diamonds. 



The quotations and abstracts given above, which outline very 

 briefly the type of eruption and describe the diamond-bearing rocks 

 of South Africa, serve as a basis for comparison with the diamond- 

 bearing rocks of Arkansas. In South Africa work has proceeded to 

 great depths — 3,601 feet in the Kimberley mine — and the geologic 

 relations are known in considerable detail, whereas in Arkansas only 

 the surface of the ground has been scratched. Nevertheless rather 

 striking points of similarity between the diamond-bearing rocks of 

 South Africa and of Arkansas are evident. 



The violence of the volcanic explosions by which the South 

 African vents were produced and the extent to which the kimberlite 

 and country rock were shattered and mixed together in the vents 

 have been emphasized. In Arkansas the peridotite was evidently 

 much shattered and the explosive violence brought to the surface 

 Paleozoic rocks that lay far beneath the surface. Injection breccias 

 that are composed of shattered country rock mixed with a small 

 proportion of volcanic material and that have the characteristics of 

 dikes have been identified in both areas. 



In many of the South African vents recurring volcanic activity 

 produced compound pipes that are formed of slightly different rocks 

 and that even bear diamonds of dissimilar character. Petrographic 

 studies have shown that in Arkansas there are at least three types 

 of rock, only one of which carries diamonds in considerable quantity. 



DIAMONDS 



Nearly all the diamonds obtained from Arkansas have been found 

 within the exposures of peridotite in the area near Prairie Creek and 

 at the Kimberlite and American mines near by, though a very few 

 diamonds have been found along streams that have washed them 

 from these areas. The mines that have produced diamonds are the 

 Ozark, Mauney, Arkansas, Kimberlite, and American. Although 

 most of the diamonds at these mines have been found on or near 

 the surface, others have been found at depths of as much as 20 feet. 



8 Op. cit., pp. 27-28. 



