SERRA DO ESPINHAQO, BRAZIL 391 



the diamond-mining town of Grao Mogol, Helmreichen (4) has given 

 an excellent description, accompanied by a geological section of his 

 route from the middle Jequetinhonha across the serra to the Rio 

 Verde. ^ According to this description and section, the Serra do 

 Espinhafo is here a broad mountain plateau, which rises abruptly 

 above lower plateaus on either side. The eastern plateau consists 

 of truncated strata of micaceous gneiss and micaschist with, near the 

 foot of the serra, intercallations (outlyers ?) of quartzite ; the western 

 one, of truncated inclined beds of grauwacke and limestone. The 

 central, or Serra do Espinhafo, plateau consists of an undulating cen- 

 tral portion of various schists, including gneiss and quartzite, with 

 patches of granite and hornblende rock, above which rise ridges of 

 quartzite which is frequently conglomeritic,^ and in which diamonds 

 occur — a circumstance that has given great importance and interest 

 to the locality of Grao Mogol. From specimens that have come to 

 hand, and from information given by two excellent geological obser- 

 vers, Drs. Francisco de Paula Oliveira and L. F. Gonzaga de Campos, 

 it is clear that (as in 1882 I predicted would eventually prove to be 

 the case), (12), the section at Grao Mogol is essentially identical with 

 the one at Diamantina, which will now be considered. 



In the neighborhood of Diamantina the range also consists of a 

 broad mountain plateau, with a mean elevation of 1,200 to 1,300"^, 

 rising abruptly above lower plateaus on either side. In the one to 

 the eastward truncated strata of gneiss and micaschist, full of quartz 

 veins that are frequently auriferous, have been leveled up to about 

 the 900"^ line by horizontal beds of soft sandstone which cover an 

 extensive area in the valleys of the Jequetinhonha and its tributary, 

 the Arassuahy, forming table-topped hills and ridges between the 

 valleys which have been cut from 200 to 300"^ below the level of the 

 plain. Hartt, who visited this region in 1865 (6), correlated these 



' This section is along a zigzag line which in part accompanies the strike of the 

 strata. Thus, while it gives a transverse section of the range, it fails to give an accu- 

 rate idea of the relative width of the different zones. 



2 Helmreichen, following Eschwege, lumped all the quartzites of the region 

 together under the name of " itacolumite, " and interpreted the pebbles in the con- 

 glomerate as concretions or segregations. For this reason, although he clearly de- 

 scribed and figured evidences of unconformability between two distinct series of 

 quartzites, he did not interpret them as such, and thus his otherwise excellent descrip- 

 tion of the region loses much of its value from a geological point of view. 



