woodworth: geological expedition to brazil an'd chile. 75 



northeastern Parana is highly suggestive of moraines Laid down by 

 land ice; but other sections in Sao Paulo and to the south and west 

 in Parana are equally clear as to their deposition by floating ice at 

 times at any rate in the sea as the presence of marine fossiliferous 

 marine shales in Parana plainly show. 



More particular problems arising in the study of these glacial 

 deposits are discussed under separate headings. 



The Striated Pebbles. — So far as present knowledge goes, striated 

 surfaces have not been found on the large boulders and block erratics 

 in south Brazil, but such ice-worn surfaces are frequently met with 

 on pebbles and fragments of rock ranging from the size of a hen's egg 

 to that of a man's head. In almost every exposure of conglomerate 

 whose pebbles fail to displa%' the well-rounded contour of water- 

 wear a few' striated pebbles may be found after a few minutes search; 

 yet of these sections I saw none in which all the pebbles and fragments 

 were striated. On the whole the proportion of striated pebbles in the 

 tillite beds is about as large as it is in the granitic till of the glacial 

 deposits of New England, but glaciated stones are certainly not so 

 abundant in any given mass as they are in the stony blue clays of 

 many localities in the Pleistocene deposits of, for instance, portions 

 of the Wisconsin moraines of the state of New York. 



The absence or apparent absence of striae from the larger blocks 

 and boulders is quite in keeping with the distribution of striae in the 

 Pleistocene of many districts and has little significance in the argu- 

 ments for or against glacial action, though in the case of the Brazilian 

 deposits striae were not earlier noted perhaps because attention was 

 given more to the blocks of striking size than to an examination of the 

 worn surfaces of small fragments of transported rock. 



Broken up striated Rock-floors. — As the glaciation and consequent 

 striation of the indurated terrane over which the ice moves proceeds, 

 the rock-floor particularly at the upstream edges of small declivities 

 in the rock-surface breaks away, so as to produce angular fragments 

 with one flat well-striated surface — that of the original floor. Such 

 pieces of rock may subsequently be striated over all their new fractured 

 surfaces but will for short journeys tend to preserve their features. 

 Thus the striated stone shown in Plate 27, the first striated fragment 

 found in Parana, presents all the ear-marks on its well worn and 

 striated side of a piece of old rock-floor even to the chatter-marks in 

 the broader and deeper groove; while on the reverse, where there are 

 two intersecting warped surfaces one is slightly scratched, and the 

 other is a much more recent fracture, still of a date anterior to the 

 final embedding of the pebble. 



