WILLIS] GENERAL GEOLOGIC NOTES 29 



to have been distinct in time, the latter succeeding the former. Thus 

 we may probably recognize an episode during which the alluvial 

 plain formed by the surface of the older Pampean was raised to its 

 present altitude above base level, an episode of humid climate dur- 

 ing which the drifts of eolian loess, the Upper Pampean, were 

 deposited. As those drifts are now cut through by the streamlets, a 

 later episode of erosion is distinguishable, which appears to coincide 

 with the present time. 



''Upper Pampean" thus gams a certain definiteness as a geologic 

 term by virtue of the physical relations in which the characteristic 

 deposits occur, and may have value in systematic classification if it 

 be limited to deposits formed during that episode of aridity which 

 preceded the present humidity. The writer has not seen enough of 

 the field, however, to know whether such a distinction is valid or not, 

 and it does not appear that the term has been thus critically applied. 



Post-Pampean deposits fall into four classes, namely: Lacustrine, 

 alluvial, dune, and marine formations, all of which are dependent on 

 topographic features as they now exist. In order to develop the 

 conditions of deposition, it is necessary to describe the topography and 

 the Atlantic coast of the pampas with reference to their origin and 

 stage of growth. We will take up first the topography of the pampas. 



The word pampa, signifying ''flat plain," describes the pampas 

 correctly only in so far as it applies to the general aspect of the surface. 

 One must look beyond the foreground in order to see the extraordinary 

 flatness of the pampas. In detail they are not flat, and yet in the 

 foreground itself there are rarely those inequalities of the surface which 

 are common m plains traversed by running streams, even where they 

 are least eroded. The absence of running water and of the landscape 

 forms which it produces is one of the most striking tliough negative 

 phenomena of the Argentine plains. The characteristic surface form 

 of the pampas is a gentle hollow or an equally imperceptible swell, 

 each of them entirely devoid of line or sharp accent and each melting 

 without distinction into the other. A horseman galloping over the 

 apparently dead level surface sinks partly out of sight like a ship 

 beyond an ocean swell and remains perhaps below the plane of vision 

 while he rides a mile or more. A rabbit startled from the grassy flat 

 is lost in sameness of color until he suddenly appears in silhouette 

 against the sky as he tops the swell and beyond it disappears. These 

 broad hollows and swells have no systematic relation to any structure 

 of the Pam})ean earths, nor to any system of drainage. They are 

 probably related to the prevailing direction of the winds which pro- 

 duced them, whether such winds be those of the present climatic 

 episode or were those of a preceding time; but if such a relation exists 

 the forms which may betray it will be discovered only by careful 



