854 THE JOURNAL OF GEOLOGY. 



sand, and a great quantity of cobbles of augite porphyry, the size of one's 

 fist at most. The big bowlders are of muscovite granite rich in tour- 

 maline, of augite porphyry, of mica schist, of soft yellow sandstone, 

 etc. The diamonds are found only in the sand containing cobbles of 

 augite porphyry. The diamonds thus far found are small, but they are 

 all of the first water. The following minerals are found with the 

 diamonds : rutile, magnetite, tourmaline, pyrope, a single ruby, and, 

 most abundant of all, limonite. Magnetite is the most characteristic 

 of these minerals. 



In regard to the original matrix of the diamonds found at Agua 

 Suja certain circumstances suggest that they have originated in a man- 

 ner analogous to those of South Africa. Hitherto it has been thought 

 that the diamonds of eastern Minas were derived from the mica schists. 

 The African diamonds, on the other hand, are from rocks of eruptive 

 origin. The following are Dr. Hussak's reasons for believing Agua 

 Suja diamonds to have an origin similar to the African ones : 



I. The absence or rarity of many of the minerals that accompany 

 the diamonds found about Diamantina. 



II. The presence of others, such as the pyrope garnets, that are 

 characteristic of the African mines, which, in a certain sense, indicate 

 the presence of eruptive rocks, but which are rare or altogether want- 

 ing in the Diamantina washings. 



III. The abundance and character of the magnetite pebbles indi- 

 cate a highly basic rock of eruptive origin, and related to the African 

 peridotite. 



On the other hand it is admitted that the abundant fragments of 

 mica schists and granite show that these have furnished materials to 

 the Agua Suja gravels, and in the absence of positive proof to the 

 contrary it is admissible that either of them may have furnished the 

 diamonds. 



The remainder of Dr. Hussak's report contains an interesting sketch 

 of the general geology of the high plateau of Brazil. The rocks of 

 the region traversed — ■ southwestern Minas and southeastern Goyaz — he 

 divides into two groups : first, the crystalline schists which consist of 

 mica schist and itacolumite. These are cut by eruptive granites and 

 are auriferous. Second, sandstones and Palaeozoic clay shales, the latter 

 enclosing gray limestones. 



Those portions of Minas and Goyaz traversed — part of the great 

 central plateau of Brazil — form a typical plateau of transgression. 



