STRATIGRAPHICAL GEOLOGY 
421 
his hypothetical mountain arcs which he calls the Pre-Patagonian 
Cordilleras. The Paleozoic positive areas are characteristically 
flanked by the arcs and in the interior of these the depressions 
areas occur. 
There are three principal negative elements along the coast. The 
extent of depression in these areas is not the same. In the south¬ 
ern two there was greater depression at an earlier age. 
Farther west there are certain negative elements, the position of 
which is not so well known, but the fact that they contain lignite 
beds of Cretacic and Tertic ages shows that a trough existed there 
during these times. 
In various ways the positive areas have influenced the character 
and phase of the sediments deposited between them in the inter- 
montane depression areas. Their changing state of denudation 
and elevation determined the limits of the epicontinental sea. 
When the sea withdrew these positive areas were the source of the 
materials of the continental deposits. Furthermore, whatever 
folding followed must have had a different effect between the 
positive! areas and in the positive areas themselves. 
A study of the tectonics of the Cretacic and Tertic sediments in 
the San Jorge basin shows two fundamentally different move¬ 
ments. First, a movement which formed saddles and troughs a 
few hundred meters across, accompanied by considerable fault¬ 
ing ; second, a folding which resulted in folds of greater amplitude, 
and which can be recognized over a wide district. In the first 
movement the force acted in a lateral direction; while in the 
second the forces were caused by the vice-like free ends of the 
mountain arc. A third phase of movement has forced the west 
end of the basin up and brought the part along the gulf down 
until the Tertic deposits are now at sea level. 
Walter R. Smith. 
Basic Tertic Conglomerate of Black Hills. South and east of 
the Black Hills uplift, in South Dakota and Nebraska, appears a 
wide-flung mantle of quartz gravels. It caps many hills and 
occupies a lower position in much of the upland about. Although 
incidentally noted by Darton and other Government field men, 
and early associated with the White River beds by Hayden, its 
exact geological age is a matter of considerable uncertainty. It 
