388 ORVILLE A. DERBY 



regions, a limestone plateau to the westward, a sandstone one in 

 the central or Serra do Espinhafo region, and a gneiss one to the 

 eastward. The last presents a remarkably level aspect, being appar- 

 ently everywhere of nearly uniform altitude, but is dotted with iso- 

 lated, abruptly rising points and knobs, as shown in the annexed 

 sketch. The soil-cap is thin, and often entirely absent, over areas 

 of several acres in extent.^ In the serra zone a horizontal sandstone 

 gives rise to a beautiful level, grassy plain occupying the summit of 

 the watershed. To the eastward this table-land breaks down abruptly 

 in precipitous cliffs, to the lower gneiss country, but to the westward 

 the slope is more gradual to the limestone plain, and across this to 

 the Sao Francisco. Above the generally level surface of the latter 

 rise irregular pinnacled hummucks and low serras. The limestone 

 is highly inclined, and in one place was seen lying unconformably 

 under horizontal beds of sandstone that stretched away southward 

 like a vast level floor. Near the western base of the serra two large 

 hills of hornstone or chert were crossed. 



From the general topographic features of the Serra do Espinhafo 

 zone it would be natural to assume that the sandstone sheet of the 

 Jacobina region should be considered as the northern prolongation 

 of that of the upper Paraguassu district, which, with its character- 

 istic diamond contents, certainly extends northward as far as Morro 

 do Chapeo, or about two-thirds of the intervening distance between 

 Lenjoes and Jacobina. In this case it has remained unfolded in 

 the northern end of the zone — a seemingly improbable hypothesis; 

 or the evidences of folding escaped Mr. Allen's observation, which 

 also seems unlikely, as his sketch confirms his description. More- 

 over, unfolded sandstones occur in the region at no greater distance 

 away than those of Lenfoes, as shown in the annexed sketch by 

 Dr. Theodoro Sampaio of a table-topped ridge, ysoi"^ high, above 



I Mr Allen describes singular holes in the rocks of this region, which are usually 

 filled with water and known as caldeiroes ("pot-holes"), and states that nearly all 

 of a considerable number examined prove to be genuine pot-holes, some of which 

 were of great size. These are probably of the same nature as the caverns, looking 

 like gigantic swallows' nests in a clay bank, seen on the granite knobs along the 

 Bahia Central Railroad. So far as could be judged from distance, these are due to a 

 peculiarly localized action of disintegration. Those seen, however, were on the sides 

 of hills and would not hold water. 



