9? BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the Pelotas plateau a large tract about Lages in Santa Catharina in 
the headwaters of the Rio Pelotas from which the trap has been 
denuded so as to leave the Triassic red beds exposed at the surface. 
This region is separated from the trappean plateau of Parand by 
the Rio Iguassti as far west as Porto da Unido da Victoria, some 
leagues west of which point the dissected trap plateaus of the south- 
ern Brazilian coastal states merge in the longitude of the Rio Parana 
into a broad sheet with little topographic differentiation. Through- 
out Santa Catharina and Rio Grande do Sul the border of the trap 
area gives rise to high escarpments capped by trap sheets overlooking 
the Permian sedimentary tract. This escarpment with its varying 
relief and declivity receives local names. South of Rio Negro it is 
known as the Serra do Espigao which attains an elevation exceeding 
4,000 feet. The main, almost unbroken, escarpment farther south and 
east is known as the Serra Geral. 
The Trap Escarpment.— This line of escarpments has been inter- 
preted as a fault cliff and as a true retreatal escarpment due to differ- 
ential erosion. Doubtless small faults intersect the trappean series 
as elsewhere in the region but the descriptions of others and my own 
observations upon the escarpment in the Serra do Espigao and at the 
head of the Rio Tuberao leave no question in my mind that the 
escarpment is the effect of differential erosion on the Permian and 
Triassic beds capped by resistant sheets of trap. 
At the head of the Rio Tuberao the Serra Geral is a typical steep- 
walled trap escarpment below the base of which erosion spurs and 
ravines of the sedimentary beds comprising the Trias and Permian 
are developed in sharp relief under the active headwater attack of the 
Rio Tuberao and its tributaries. 
Inland in the region of the Serra do Espigao the alignment of the 
escarpment is less simple in its curvature. Siemiradzki in a section 
reproduced by Suess in “ La face de la terre’ assumes a normal fault 
along this line as he does also in the case of the “cuesta” of the 
Devonian sandstones but without structural evidence, and here 
also the topographic development of the rocks is simply a matter of 
secular denudation as shown in the geological section drawn by 
Derby and published by Branner in his Geologia elementar. 
Siemiradzki represents the Serra do Espigao as a ridge of lava piled 
up above a dike. The northwestern spur of the Espigao forms a high 
terraciform ridge standing out above the trap plateau south of the 
escarpment but the table-like masses of which the ridge is composed 
indicate that it is a portion of the trap sheets left standing out in 
