1S7P.I J-'* [Derby. 



ter and position to those just described, but so far they have yielded no fos- 

 sils, that will serve for their classification, and they have been referred to 

 the Tertiary solely on account of their stratigraphical position. The only 

 Tertiary fossils known from this region are the fossil leaves of Tonanlins, 

 and the fresh and brackish water mollusks of Pebas and other localities in 

 Peru. These, however, occur in lignitiferous beds, quite different from 

 those now under consideration, and the relation between the two has never 

 so far as I am aware, been satisfactorily determined. The only division 

 that can at present be made in the region of the Lower Amazonas, is between 

 the beds of the high table-lands, and those of the lower plains about Para 

 and eastern Marajo. These last, consisting of abrupt alternations of coarse 

 and fine sandstone, generally ferruginous, along with colored clays, are 

 certainly more modern than the former, and belong to the later Tertiary 

 or the Quarternary. 



During the deposition of the Tertiary, there were considerable movements 

 of depression and subsequently of elevation, but these movements were, 

 as far as is known at present, unaccompanied by disturbances of the strata 

 or eruptions of igneous rocks. 



After the elevation of the Tertiary table-lands, began the alluvial deposits 

 of the varsea. They consist, according to circumstances and localities, of 

 sands or clays, or a mixture of the two, a yellowish structureless clay pre- 

 dominating, often having above it a bed of black clay, impregnated with 

 vegetable matter. Part of this deposit was without doubt formed in an 

 estfuary, while the river was taking possession of the bed prepared for it ; 

 but it is now impossible to distinguish the estuary deposits from those that 

 are purely fluvial. The proofs of the estuary condition are not so much in 

 the characters of the deposits, as in the form of the tributary valleys, which 

 are widened in a manner that can only be explained by the action of the 

 tides. 



With the formation of the varsea, the geological evolution of the 

 valley of the Amazonas terminated. We cannot in this place enter 

 into a consideration of the interesting phenomena, illustrative of Geology 

 and Physical Geography, of which the varsea is the theatre. To 

 witness, close at hand, the operation of many of the processes of which 

 these sciences treat, and which have given form and character to 

 the surface of our planet, I know of no region equal to the Amazonas. 

 Between the water and the land, the river and the varsea, there is a 

 constant conflict. Islands are formed and destroyed, or floated bodily 

 down stream, by the continual process of destruction at one end, and of 

 formation at the other ; lakes, furos and parana-merins are being formed, 

 to be again filled up ; tributaries extend themselves into the territory 

 proper to the main river, or this throws out one of its lateral channels, to 

 appropriate to itself a part of the valley of a tributary. The conflict, how- 

 ever, is unequal ; the force of the river, irresistible as it is in its great floods, 

 is spasmodic in its action, and can be met by a weaker, more constant one, 

 such as is afforded in aid of the growth of the land, by the vegetative force. 



PROC. AMER. PHILOS. SOC. XVIII. 103. W. PRINTED MARCH 11, 1879. 



