1872.) 153 [Hartt. 
particularly on the theory of the glacial origin of the Basin, 
propounded by Prof. L. Agassiz. 
Prof. Hartt stated briefly Prof. Agassiz’ theory,—that the valley of 
the Amazonas is occupied by a series of beds, in the following as- 
cendine order, viz: first a bed of coarse sand, second, a series of 
clays, and thirdly, a heavy mass of sand-stone several hundred feet 
in thickness, all of which were deposited in an ice-laden, fresh water 
lake confined behind a gigantic moraine which stretched across the 
valley, several hundred miles east of the present mouths of the Ama- 
zonas. Prof. Agassiz supposed that by the bursting of the barrier 
the water of the lake rushed violently out, denuding away the sand- 
stones from over the greater part of the valley, leaving only a few in- 
sienificant patches forming the table topped hills on the left bank of 
the Amazonas, between the Rivers Xingu and Tapajos. The dimin- 
ished waters of the lake were supposed to have deposited a series of 
clays over the valley, and in these clays Prof. Agassiz claims to have 
found eratics at Ereré. Prof. Agassiz based his theory on his exam- 
ination of the Serra of Ereré and its vicinity, taking that serra as a 
type of the system of table-topped hills. Prof. Hartt showed that 
the Serra of Kreré is composed of a great thickness of an ex- 
tremely hard sand-stone, which dips at an*angle of about 15° to 20° 
to the southward. At its base on the north is an immense plain, 
composed of quite horizontal strata. The lowest are well bedded 
chertz rocks, on which lie a series of shale, white, mottled and occa- 
sionally black or bluish, succeeded by thin beds of sandstone. These 
beds were supposed to be drift by Prof. Agassiz.” Prof. Hartt found 
them full of well preserved fossils, Spirifer, Leptocelia, Strepto- 
rhynchus, Chonetes, Nuculites, Paleoneilo, Tentaculites, Dalmanites, 
&c., indicating a lower Devonian horizon. ‘These beds are traversed 
by a perfect net-work of trap dykes, which by decomposition have 
given rise to numerous boulders. As these Devonian beds, quite undis- 
turbed, abut against the base of a high ridge whose strata are in- 
clined, the conclusion seems inevitable that the Serra of Ereré is 
precarboniforous in age. Even if it be newer than the Devonian 
rocks just described there is no reason for considerirte it to be of 
glacial orizin; moreover the serra does not belone to the system of 
table-topped hills, for Prof. Hartt found the Serra of Parandquéra, 
the most important of these hills, to be composed not of the sand- 
stone as Ereré, but of a series of soft clays, clayey sands and sandy 
clays, well bedded, horizontal, and probably of Tertiary age. Evi- 
