SERRA DO ESPINHAQO, BRAZIL 393 



that the sandstone is the source of the diamonds that characterize 

 the region. A number of the workings are in decomposed con- 

 glomeritic portions of the sandstone sheet, and at Grao Mogol 

 diamonds have been found in the sound rock as well. 



The southern section of the range, forming the head of the Rio 

 Doce basin, being in a region of very abundant rainfall, is much 

 more deeply eroded than those above described, and along the usual 

 lines of travel, which are parallel to its trend or cross it in deep gaps, 

 its tectonic features are not clearly apparent. The central zone is 

 occupied by great ridges, or blocks, of quartzites, or quartzitic sand- 

 stone, rising to an elevation of 1,500 to 1,900™, and which, being 

 comparatively barren of gold and of difficult access, are almost com- 

 pletely unexplored. These seem to represent a great sheet like that 

 of the Jequetinhonha and Paraguassu sections, which on the lower 

 grounds has been completely swept away, or so denuded as to be 

 difficultly recognizable, especially as a very similar rock occurs as 

 a member of the underlying formation. The latter consists of a 

 great series of essentially argillaceous and micaceous schists, with 

 intercallations of flaggy micaceous quartzites, ferruginous quartz- 

 ites (itabirites), and limestones, which is frequently gold-bearing. 

 In the bottom of some of the valleys gneiss, micaschist, and granite 

 appear, and these are the predominant rocks in the region to the 

 eastward of the serra, which is, however, so imperfectly known that 

 the eastern margin of the Serra do Espinhafo zone cannot be satis- 

 factorily traced. The western margin, as in the Jequetinhanha sec- 

 tion, is abrupt and overlooks a similar region of truncated sedimentary 

 rocks, which, however, throughout a considerable part of the section 

 is separated from the base of the serra by a zone of granite and gneiss. 



From the above sketch it appears that the Serra do Espinhafo 

 consists of a basement complex of metamorphic and eruptive rocks 

 overlain unconformably by one or more series of sandstone strata that 

 have been disturbed by a system of northward-trending folds, whereas 

 in the mountainous region of southeastern Brazil, of which it is an 

 offset, the dominant trend is northeastward. In the southern sec- 

 tions (Jequetinhonha and Doce) the orographic movement producing 

 the folds has also produced a certain amount of metamorphism in 

 the form of shearing and granulation, accompanied by the develop- 



