318 A. P. COLEMAN 
_ southwest from Buenos Aires brings one to the small station among 
the hills, after passing a vast stretch of prairie-like pampas with few 
or no outcrops of rock. ‘The Sierra rises as rocky ridges with deep 
valleys between, one of them followed by the river Sauce Grande 
and others by its tributaries. 
The railway crosses the river just south of the station and 
follows up the valley of a small stream in the Arroyo Negro. The 
best exposures of tillite are found in the railway cuttings along the 
Arroyo within seven kilometers of Sierra de la Ventana, and these 
will be described first. 
The unweathered tillite is dark, bluish gray and entirely different 
in appearance from the usually red or brown and much-decayed 
tillite of Brazil. The rock is hard and shows some slaty cleavage, 
and the stones scattered through it are often a little squeezed or 
broken and slightly step-faulted. The weathered tillite is greenish 
or yellowish and crumbles somewhat readily, setting free the 
inclosed stones, but from the unweathered rock it is difficult to 
extract them unbroken. The fresh tillite is very like that from some 
outcrops of the Dwyka in South Africa, where the rock has under- 
gone squeezing and distortion in mountain-building operations; 
and it closely resembles the Huronian tillite of Cobalt and might 
easily be taken for it in hand specimens. 
The pebbles and bowlders inclosed include several species of 
rocks, granites and hard sandstones being commonest. They are 
seldom more than half a meter in diameter and have the character- 
istic shapes of glaciated stones. A considerable number have well- 
striated surfaces and are typical products of ice action. 
In some of the cuttings cross-bedded quartzite and more or less 
water-formed conglomerate occur also, apparently interbedded with 
the tillite; and in several places quartzite overlies the tillite con- 
formably. The base of the tillite was not seen in the railway 
cuttings, and a search was made for it to the north, where a small 
stream flows toward the Sauce Grande, but in vain. On this stream 
the tillite has been squeezed into schist conglomerate with a marked 
cleavage, reminding one of the Temiscaming and Doré conglom- 
erates of Ontario. A search still farther north showed no solid 
rock for several kilometers until the base of the northern range 
