Diamond Regions of Eastern Minas Geraes, Brazil. 215 



analyses in which the alkalies were determined, only 67 (12 

 per cent) have over 5 per cent of potash and soda ; 99 (18 per 

 cent) over 4 per cent, and 177 (30 per cent) over 3 per cent. 

 In the 87 that are given as slate,, shale or shale-clay, the pro- 

 portion having over 5 per cent is the same (11 = 12 per cent), 

 but below this figure it rises rapidly, the numbers being 22 (25 

 per cent) with more than 4 per cent of alkalies and 62 (72 per 

 cent) with more than 8 per cent. 



In the low proportions of alkalies the difference above noted 

 may reasonably be attributed to the greater amount of water 

 in the earthy types, but this explanation will not apply in the 

 comparison of the shales with the Minas schists, which, con- 

 sidered as original elastics (as a large, and probably the greater, 

 part undoubtedly were), appear to have been uniformly richer 

 in alkalies than the normal clay deposits of more recent times. 

 If this conclusion is correct, these ancient argillaceous deposits 

 must either have been composed quite uniformly of compara- 

 tively sound felspathic or micaceous material rather than of 

 kaolin, or in some way the original proportion of alkalies must 

 have been increased. The hypothesis above given that some 

 of these schists are of eruptive origin will in part explain the 

 predominance of micaceous rocks in the region, but, for the 

 present at least, there are no good reasons for supposing that 

 these constitute more than a small fraction of the whole. The 

 case of J^o. Ill shows that a high proportion of alkalies is not 

 necessarily indicative of eruptive origin and a number of other 

 highly micaceous rocks have been washed with the same result 

 of a residue with well-characterized allothigenetic elements. 



An excess of alumina over that required by the alkalies to 

 form mica, and which goes to form aluminous silicates, seems 

 also to be a common feature of the schists of the region, as 

 cyanite js a very common mineral both in the rock outcrops 

 and in the loose material of the surface, as well as an element, 

 often predominant, of the concentrates of the gold and diamond 

 placers. In general, however, it appears to come from the 

 more quartzose rather than from the more micaceous members 

 of the series and in these cases it may be presumed to represent 

 kaolin in original clastic sediments. Other aluminous silicates 

 that resist decay so as to appear in the washings, as staurolite, 

 are not at all prominent. From some of the above analyses it 

 may be suspected that pyrophyllite will prove to be a common 

 and widespread element, but from its liability to be confounded 

 with mica it can only be detected in perfectly sound rocks, that 

 are rare in the region. .As above indicated, this excess of 

 alumina may be attributed to the normal presence of kaolin in 

 original elastics, or to the leaching of original eruptives. 



