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bulletin: museum of comparative zoology. 



stones occur at the top of the bed embedded in the overlying sandstone. 

 No glacial striae or facets were seen on any of the exposed pebbles. 

 Besides the sandstone pebbles, I saw one of micaceous quartzite, a 

 crumpled phyllite, a whitish chert, a red argillaceous sandstone, a 

 decomposed granite with reddish feldspar, a silicious schist, iti toto 

 a variety of rocks indicating the derivation of at least a small portion 

 of the debris from the Pre-Devonian terrane. The pebbles could 

 hardly have been transported far by water action alone, because the 

 sandstones pebbles are of a sort which do not wear well in any stream 

 journey. I estimate their journey as stream pebbles to have been 

 tens of miles rather than hundreds of miles. The assemblage of these 

 pebbles in a well-stratified bed between layers of a coarse-grained 

 sandstone leaves no doubt of water action. A few feet of sandstone 

 separates this bed from the one above, in which the sandstones peb- 

 bles again formed the predominant "constituents but were noticeably 

 more rounded than the crystalline pebbles. Dr. I. C. White men- 

 tions in his Report a bed of boulders at Ponta Grossa and a deposit 

 containing fossil wood, but I saw none, nor were such deposits known 

 to the Geological Survey staff at the time of my visit. 



Tilliie Bed at Conchas: — Conchas lies on the north side of the Rio 

 Tibagy about four leagues west from Ponta Grossa. On the south 



of the village a small quarry 

 was opened some years ago 

 in a grayish somewhat in- 

 durated stony clay bed, a 

 boulder-clay phase of the 

 tillite beds. The scattered 

 pebbles consist of silicious 

 rocks and rarely a granitic 

 pebble. The bed fractures 

 with a giant ball structure 

 (Fig. 15). No striations were seen on the pebbles. The mode of 

 occurrence of the pebbles seems best explained by dropping from 

 floating ice and probably the clay with its sand grains of irregular size 

 originated in the same manner. The rock when exposed to the weather 

 breaks down by a process of checking and the opening of ragged 

 fractures into smaller and smaller blocks so that it is valueless for 

 building stone. 



This bed o^■erlies the sandstones with waterworn pebbles at Ponta 

 Grossa, and recalls in its lithological characters the beds on the south 

 of the Rio .Jaguaricatu east of Sengens and also the beds southeast 

 of Rio Negro. 



Fig. 15. — Fracture of the tillite bed at Conchas. 



