AND RIO GRANDE DO NORTE, BRAZIL. 15 



and forms most of the valley floor, as well as the low hills well up 

 toward the contact. As a general thing, however, the margins of the 

 basin, or the part nearest the contact with the crystalline rocks, are 

 composed of a coarser sandstone than the parts further down. In 

 fact, the part nearest the contact is often a conglomerate, having 

 small quartz pebbles, some the size of a hen's egg, imbedded in the 

 sand. As a rule these sandstone beds dip gently to the south or a 

 few degrees to either side of south. There is also evidence of a small 

 syncline near the southern boundary of the valley. This syncline was 

 shown near Acauan and again near Sao Joao in relatively the same 

 position. However, it is small and is not likely to prove to be of any 

 econornic importance. The reddish and yellowish sandstone is inter- 

 bedded with a likewise reddish clay. This clay covers so large an 

 area that it must be taken into consideration. In appearance it is 

 very much like the sandstone but of a finer grain. In several places 

 in the valley it was found plainly interbedded with the sandstone. 

 It is most common in the lower part of the basin, that is, it is found 

 from one end to the other but in any one cross-section of the valley 

 the clay would be more likely to be found near the river or in the 

 lower part. The whole of this deposit of sediments is intergraded 

 between a coarse sandstone of a conglomeritic nature, a medium- 

 grained sandstone, a fine-grained sandstone of a partially clayey 

 nature, and a typical clay. At times one may find all these dififerent 

 grades of rocks interbedded in one place. 



So far as I know no fossils have ever been found in these rocks. 

 The sandstone very much resembles that which underlies the lime- 

 stone in the coastal belt, and it is provisionally referred to the Cre- 

 taceous. There is a possibility of its being connected with the sand- 

 stone of the Serra Araripe. There is also a possibility of its having 

 been connected with the sandstone of the coastal belt. Indeed, it is 

 difficult to account for its presence on any other hypothesis. 



The Serras of Martins, Porto Alegre and Joao do Valle are a 

 series of sandstone capped mountains which rise abruptly to a height 

 of 650 to 700 meters. The last named of these was not visited by the 

 writer. The first two have a layer of hard, quartzitic, coarse- 

 grained sandstone which attains a maximum thickness of about 50 

 meters, and which lies horizontally on the crystalline rocks. No 



