ON THE GENESIS OF THE DIAMOND 1 25 



characteristic is the presence in great abundance of magnetite 

 and of a magnetite rock, which Dr. Hussak has succeeded in 

 tracing to a special magnetite-perofskite type found by him near 

 Catalao, in the state of Govaz. 1 



This last element of the diamantiferous bed cannot be 

 referred to any of the known rocks of the region, but it points 

 in the same direction as the above mentioned eruptive elements 

 of the limestone and clay beds that are known to occur in 

 the region and that may be presumed to extend to the 

 immediate vicinity of Agua Suja. These elements, pyroxene, 

 perofskite and magnetite, suggest a type of basic eruptive pass- 

 ing into an iron ore such as has actually been met with in the 

 Jacupiranga district of the state of Sao Paulo in genetic relations 

 with various nepheline-bearing rocks, the whole constituting a 

 typical volcanic series. 2 As a somewhat similar volcanic center 

 is known at Caldas 3 at no great distance from the Agua Suja 

 region, there is a reasonable probability that another one may 

 exist in the immediate vicinity, and that it may have furnished 

 the problematic material of the diamantiferous bed. This last 

 is not clearly referable to the present drainage conditions of the 

 country and is very likely to prove to be an ancient conglomer- 

 ate, or breccia, possibly in relation with the eruptive manifesta- 

 tion that is presumed to have contributed to its elements. 



The Agua Suja occurrence thus offers a certain number of 

 analogies with those of the Kimberley district which are entirely 

 lacking in the other Brazilian localities, so far as they are known. 

 It is especially to be noted that the absence of these analogies 

 is as conspicuous at the nearest locality, Bagagem, only about 

 twenty miles distant in the same river basin, as at any other. 

 The country rock both at Kimberley and Agua Suja is hori- 

 zontal, of approximately the same age (late palaeozoic or early 

 secondary) and with intercalated sills of trap of very similar 

 character and composition, but which in both cases has no 



1 Neues Jahrbuch, 1894, H> P- 2 97- 



2 Derby: Am. Jour, of Sci., XLI, 1891, p. 311. 



3 Derby : Quart. Jour, of the Geol. Soc, 1887, p. 457. 



