4i8 E. C. HARDER AND R. T. CHAMBERLIN 



farther north in scattered localities they contain concentration 

 deposits of brown iron ore and manganese ore. In the United 

 States these deposits have been referred to the Tertiary or late 

 Cretaceous. 



From the great predominance of quartzite pebbles it must be 

 supposed that the materials composing these soft conglomerate 

 deposits came largely from the Caraga quartzite. The immediate 

 source of the igneous pebbles present in some localities, however, 

 is less certain. These may once have been laid down in con- 

 glomerate beds in the Caraga quartzite and have been worked over 

 again later, or they may have been derived from the rocks of the 

 igneous area adjacent to the Espinhago sedimentary belt. The 

 Caraga formation in this district is a well-consolidated quartzite 

 of medium-grained uniform texture. Conglomerate beds are rare, 

 and though scattered pebbles are frequent, these are almost invari- 

 ably of vein quartz. There is a possibility, however, that pebbles 

 of igneous rock may be present locally. 



Accepting the supposition that the material composing the 

 conglomerate was derived largely from the Caraga quartzite, one 

 would naturally look for the origin of the diamonds in the same for- 

 mation. In the Diamantina district, diamonds have never to our 

 knowledge been found in the Caraga quartzite. It is, however, 

 possible that they may occur so widely scattered that it is only by 

 repeated concentration such as has occurred in the conglomerate 

 deposits that they become noticeable. In the Grao Mogul district 

 of northern Minas Geraes about 200 kilometers to the northeast 

 of Diamantina, diamonds are reported to be found in hard quartz- 

 ite^ from which they are freed by blasting. However, it is not 

 known whether this is the Caraga quartzite or whether it belongs 

 to some other formation. 



In conformity with the generally accepted theory of their origin, 

 the diamonds may have been derived originally from intrusions 

 of igneous rock at a distance from the present areas of concen- 

 tration. Disintegration and decomposition of the inclosing rocks 

 would have resulted in the freeing of the diamonds, leaving them 



' O. A. Derby, "Modes of Occurrence of the Diamond in Brazil," Am. Jour. Sci., 

 3d Ser., XXIV, 39. 



