76 bulletin: museum of comparative zoology. 



Although no glaciated rock-floor has yet been found in south Brazil,. 

 pebbles of the type above described point to the one-time existence of 

 such a rock -floor though it is no longer to be seen. In this particular 

 instance that of a pebble from the tillite beds on the Jaguaricatu the 

 rock is a reddish brown ^ fine compact argillaceous sandstone, with a 

 perceptible clayey odor when breathed on, and carrying minute scales . 

 of muscovite. 



Crushed and Blunted Rock Fragments. — Striated pebbles or rock 

 fragments with or without flattened sides or "soles" are commonly 

 regarded as the most characteristic molar constituents of moraines 

 directly due to glacial action; but there is another type of pebble in 

 ice-laid moraines which is equally peculiar to the process of erosion 

 and transportation; that is the pebMe with blunted and snubbed 

 ends with or without striae. This kind of pebble is usually rather 

 elongate and oftener displays its fractured and splintered surface at 

 that extremity which has the smallest cross-section. Such fragments 

 are not uncommon in the glacial drift of North America and I have 

 found them in the tillite of Parana in the beds along the Rio Jaguari- 

 catu. One example quite characteristic in every feature is illustrated 

 in Plate 27, fig. 3. 



The small intersecting surfaces wdiich give rise to the beveled appear- 

 ance of the subpointed portion of the periphery of this rock fragment 

 are surfaces of fractures produced by the riving off of spalls of the 

 rock through pressure applied at points along the major perimeter 

 of the pebble. In the case of this pebble, there are three such larger 

 fracture surfaces and each of the later fractures was followed by a 

 repetition of the pressure at approximately the same point so as to 

 force a smaller spall with a subconchoidal fracture. At some stage 

 in the same process after the intermediate surface was produced by 

 fracture, the wedge-shaped point of the pebble was broken squarely off. 

 All of this fracturing was accomplished previous to the final embedding 

 of the pebble in the tillite bed. Owing to the concavity of the fracture 

 surfaces, they escaped striation, yet some slight scratching took place • 

 on the two larger surfaces. 



Such fracturing is apparently clue to the forcing of pebbles against 

 the bed rock or against other rock fragments in the ice or by their 

 being caught under boulders so as to have a great weight of ice con- 

 centrated upon them when they in turn are in contact with the bed 

 rock. 



1 The color of the dry isolated rock is close to Orange 130, Klincksieck.et. Valette: 

 Code des coleurs (Paris, 1908^. 



