134 AMAZONIA AND LA PLATA. 



following vears Henry Koster traversed the coastlaiids between Recife and 

 Maranhâo. In 187Ô Wells, starting- from Carolina on the ïocantins, crossed the 

 mountainous zone, and reached Maranhâo by the Rio de Grajahu valley. The 

 coast has also been carefully studied by Vital d'Oliveira and later by Mouchez, 

 both of whose charts serve as bases for the still-defective maps of the interior. 



Physical Features — Geology. 



The coast ranges between the Tocantins and the S. Francisco basins constitute 

 no continuous chains with regular watersheds ; they are evidently the remains of 

 elevated plains eroded by running waters, although the crests may still enable 

 geologists to divine the original structure of the ravined plateaux. The loftiest 

 ridges appear to be the Serra do Piauhy and the Serra dois Irnicxos, which 

 dominate the course of the S. Francisco on the north-west. These crests, running 

 south-west and north-east, may be regarded as the edge of a plateau, another 

 edo-e of which is formed in the south-west by the Serras Mangabeiras and 

 Gurgueii. Some of the peaks exceed 3,000 feet, although the highest measured 

 by AVells between the Tocantins and the streams flowing to the Gulf of Maranhâo 

 only attained an elevation of 2,100 feet. 



Tjittle is known of the geological structure of these uplands. The escarp- 

 ments inclined towards the S. Francisco consist of archœan rocks analogous to 

 those of Canada, and to the same formation belong the heights on the projecting 

 coastland between Ceara and Alagoas ; but farther west these primitive rocks 

 underlie calcareous strata of the chalk ages. The whole of the Upper Parnahyba 

 valley is occupied by these formations, while farther north, parallel with the coast, 

 follow sandstone terraces like those of Amazonia, doubtless dating from the same 

 period when arenaceous sediment was deposited on the bed of a vast fresh-waier 

 Mediterranean. 



At that time the Amazonian gulf formed dry land, and the same agencies are 

 still at work eating away the present seaboard. Between the Para and Maranhâo 

 estuaries, a distance of about 300 miles, land and water are continually battling 

 for a tolerably broad belt of creeks, inlets, islands, reefs, channels, and lagoons, 

 intermingled in endless confusion and shifting with every tide. Here the bore 

 rushes in with tremendous fury, at times with a velocity of six miles an hour, a 

 veritable tidal cataract tearing the beach into shreds and sweeping away all 

 obstacles. But in this ceaseless struggle the advantage remains with the ocean. 

 Along the strand beds of marine organisms are found superimposed on shell 

 mounds of lacustrine origin ; the aquatic mangrove is encroaching on the land 

 flora, and here and there clumps of palms are seen already invaded by the surging 

 waters. 



Rivers. 



Numerous streams descend from the hills and plateaux of the Atlantic water- 

 shed ; but no river, not even the Parnahyba, rivals the great Amazonian affluents 

 in the length of its course. The Gurupy, flowing between the States of Para and 



