1879.] lul [Derby. 



"When the continent was once more brought above water, the plateaux, 

 leveled by their new acquisition of strata, first rose ; but, by and by, the 

 present watersheds, joining the great plateaux with the Andes, came above 

 water and the Amazonian valley became a Mediterranean, communicating 

 eastward with the Atlantic by a narrow strait. The soft Tertiary beds of 

 the province of Para were rapidly denuded by the action of the sea during 

 the rise of the land. Probably, while Guiana existed as an island, the 

 Amazonas felt the influence of the equatorial current, which may have 

 aided in carrying away the results of denudation. In the end, the Ter- 

 tiary beds were completely swept away over an immense tract of count ry; 

 the Serras of Paru and the similar mountains to the northward were left 

 as monuments of their existence. * * * While the Tertiary sheet was being 

 denuded away, the streams from the highlands were cutting for themselves 

 valleys through the same beds, and these, forming estuaries, were widened 

 to a greater extent than it would have been possible for the streams them- 

 selves to have done. During this epoch of denudation deposits were 

 formed, not only in the interior sea, but also in the gulf into which it 

 opened to the east. * * * As the rise continued, the interior sea, now 

 shallowed by much sediment and freshened by the tribute of a thousand 

 streams, was rapidly narrowed in area, and the river Amazonas, properly 

 speaking, which hitherto emptied into a lake at the foot of the Andes, 

 began to extend its channel, following the retreating waters." 



The above quotation explains clearly the origin of the varzea, of the low 

 plains of Para, and of the higher plains of the interior of the province. 

 In the hilly regions the inclined beds of the formations older than the 

 Tertiary, including the Cretaceous, the Palaeozoic and the Archean, appear 

 in virtue of the denudation of the overlying Tertiary sheet. 



The rocks of the ancient islands, the first lands that appeared in the 

 ocean in which the continent was forming, have been profoundly meta- 

 morphosed, being converted into granite, gneiss, quartzite and metamor- 

 phic schists, and by reason of this, the extent of these islands may be 

 approximately determined by the study of the distribution of the meta- 

 morphic rocks. Those of the north appear in the high mountains of Gui- 

 ana, along the boundary between Brazil and Guiana and, decreasing in 

 elevation towards the south, extend to a line that, beginning near the At- 

 lantic and the mouth of the Amazonas, in about latitude 1° N., extends a 

 little south of west, to the confluence of the Rio Branco and Rio Negro, 

 between latitudes 1° and 2° S. Along this line, which represents the an- 

 cient coast, the metamorphic rocks are in general only exposed in the 

 valleys, by the denudation of the Tertiary beds. To the west of the 

 mouth of the Rio Branco the}' extend to, or beyond, the upper Rio Negro. 



On the Brazilian side, the metamorphic rocks only form high mountains 

 in regions far distant from the Amazonas ; but they are met with under 

 the other formation in the greater part, if not in all the elevated portions 

 of Brazil. In the Amazonian region, they form the rapids of the rivers 

 Tocantins, Xingii, Tapajos and Madeira, the line of exposures passing the 



PROC. AMER. PHILOS. SOC. XVIII. 103. U. PRINTED MARCH 10, 1879. 



