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 pebble 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 which 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 due 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. 
! The color of the dry isolated rock is close to Orange 130, Klincksieck et Valette> 
©dde des coleurs (Paris, 1908). 
Pi Son 
ra] 
