456 JENKINS— GEOLOGY OF THE [May 29, 



Alluvium. 



The only fertile land of the country is the alluvial deposits. 

 They occur in the wide flat valleys as a black, sandy loam containing 

 a good deal of plant matter. These deposits have been carried down 

 by the rivers and washed in by the sea. In some places as in the 

 river by Natal, this silting up has not yet been finished. Here man- 

 groves act as a sieve for the sediments. In some places they have 

 only left a comparatively small passage for the water, taking up at 

 least four fifths of the entire valley, which is about six kilometers 

 wide. Their many spreading roots retard the flow of the heavily 

 laden water, which drops its load and thus fills up the channel. 



Some marine shells were excavated from an irrigation canal 

 made in the center of the valley of Rio Ceara-Mirim, about ten 

 kilometers northwest of Extremoz and thirteen kilometers from the 

 coast. From their preservation they look as if they were of Qua- 

 ternary age. They occurred about a meter underground. The 

 valley is very low at this point, said to be two meters above sea 

 level. This shows that the sea must have extended up to this 

 point, probably in a long narrow channel filling the river valley, and 

 since then has been silted out, and possibly the country has been 

 raised. 



Borings for wells as far up as Carnahubinha bring up shells of 

 the little rock oyster, Ostrea equestris, in great abundance. One 

 well was sunk in the alluvium northwest of Carnahubinha, half a 

 kilometer from Rio Jundiahy and at the depth of twenty-one meters 

 a ledge of these oyster shells was struck. An Indian hammer head 

 was also dug up from a depth of three meters in the alluvium, near 

 the river at Carnahubinha.* 



This is a deposit of more recent age; in fact the deposition may 

 be observed now. 



The most interesting thing about the alluvial material is its ex- 

 treme depth. It extends below sea level. In fact all the valleys 

 show this sunken condition, for they enter the ocean below sea level. 



* Information obtained from Mr. John Charles Smith at Carnahubinha, 

 Natal, Rio Grande do Norte. 



