92 bulletin: museu.m 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 Parana by 

 the Rio Iguassii as far Avest as Porto da Uniao 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 Xegro 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 do'es 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 puljlished 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 



