WOODWORTIi: GEOLOGICAL EXPEDITION TO BRAZIL AND CHILE. ii 



Where this type of pebble occurs on the surface of the ground at the 

 present day as it does in the ghiciated regions of eastern North America 

 it might possibly be interpreted as owing its conchoidal fractures to 

 the work of aboriginal man, but when firmly embedded in the glacial 

 till of that district or in the tillite beds of Brazil no one would presume 

 to connect it causally with human art. These crushed and bruised 

 rock fragments with their sides well striated are common in south 

 Brazil and along with other striated pebbles argue for the crushing, 

 bruising action of a thick body of ice such as a glacier would afford. 

 Even where stones of this type of contour occur isolated in fine shales 

 into which they have been dropped from floating ice, the evidence as 

 to their original handling by glacial action is equally good. 



Classification of Lithified Glacial Deposits and Derived Sediments. — 

 The lithological classification of sediments takes no account of genesis, 

 its names, somewhat more carefully defined than in common usage, 

 express ideas concerning the size of constituent particles, as in the 

 terms conglomerate and sandstone; or designate vaguely a mode of 

 fracture, as in shale with an understood composition of particles too 

 small to be distinguished by the unaided eye. This simple primitive 

 classification embraces all lithified glacial deposits when used with 

 proper qualifying terms. The glacial deposits, so far identified, have 

 given rise to two independent terms, boulder-bed and tillite, the first 

 of which is con\-eniently vague, except for its reference to boulders, 

 while the second covers a wide range of commingled rock fragments 

 and particles having this in common that they were deposited by the 

 agency of glacial ice. The study of modern glacial deposits would 

 lead us to expect among ancient glacial deposits the lithified counter- 

 part of each product of glacial action and the glacio-natant waters. 

 Tillite, as consolidated till, would naturally be applied to all un- 

 stratified, unassorted deposits due to the direct agency of a glacier. 

 The term thus is applicable to the rock of fossil moraines whether 

 frontal, or ground-moraine, and to fossil drumhns. These varieties 

 of tillite, since they are distinguished by topographic form, will not 

 in the nature of the case take petrographic designations. The same 

 remark applies to the assorted glacial gravels and sands forming the 

 group of kames, eskers, and proglacial deltas, or gravel- and sand- 

 plains. For their petrographic designation there is no distinctive 

 term correlated with till and tillite. The coarser deposits are in- 

 cluded in the conglomerates and the finer among the sandstones. 

 The glacial rock-flours or clays, normally unweathered, finely divided 

 clastic materials of complex mineralogical composition, often feldspa- 



