woodworth: geological expeditiox to brazil and chile. 21 



to a deep broA^Ti earthy mass, and in small patches I saw reddish, and 

 in one place, greenish clay. Bales of spherical separation appeared 

 here and there and often the road led the mules between large angular 

 blocks which bestrewed the hard trappean surface. The contrast 

 between this thin coat of weathered trap and the deep beds of de- 

 composition forming the terra roxa in Sao Paulo is very striking. 



The Brazilian pine, Araucaria, occurred in small patches here and 

 there and many young plants pointed to favorable conditions of growth 

 for the species. An occasional Mate tree appeared along the trail, 

 evidently due to the droppings from some passing caravan. From 

 this high plateau there was a good view of a high trap range of tabu- 

 lar outline in the distant northwest, the prolongation of the Serra do 

 Espigao extending towards Porto da Uniao. The wooded surface 

 of the range concealed all the rocks, but the triple terraces of the mass 

 presumably signify a three-fold division of the trap sheets of which it is 

 composed. Between our position at Corisco and this tabular moun- 

 tain there lay an extended lower surface, the deeply dissected basin 

 of the Rio Correntes. 



Along the mule path, we passed several small grassy pools, occupy- 

 ing depressions partly enclosed in the trap. In at least one instance a 

 pool lay on the upper side of the road and the water was held in by a 

 barrier of mud and gravel accumulated in the road by wash from the 

 descending grade on either side to the sag by which the drainage 

 normally overflowed. The surrounding gramineous plants displayed 

 the brown color of winter. The fine green grass of one of the pools 

 had attracted to it a domestic horse which stood up to his knees 

 feeding, evidence that the bottom was floored with probably the same 

 stiff residual clay which later I saw in an excavation in one of these 

 basins. On this monotonous succession of trap uplands of nearly 

 uniform structure the trivial relations of the life which found a place 

 upon them became matters of more than passing interest. On the 

 muddy bottom of the rivulet which flowed past the evening camp I 

 found a small fresh-water mussel resembling Unio crawling along 

 with the open edges of the valve down so as to leave a deep narrow 

 groove in the mud. The trail was sinuous and ended in a burrow 

 where the mollusc pushed under a cover of mud. I should have 

 assumed, had I not seen this animal at work, that the trail, as have 

 been so many found in the fossil state, was to be ascribed to some 

 gasteropod. 



August 23rd. — The morning broke cloudy with rain threatening, 

 for in this latitude on the trap plateau the distinction between a dry 



