WILLIS] GENERAL GEOLOGIC NOTES 23 



The basal stratum of dark-brown, often sandy earth, marked by 

 compactness and homogeneity and frequent stratification, is a con- 

 tinuous formation, which has the characters of eoUan material redis- 

 tributed by waters. The writer has seen very similar deposits in 

 China in the banks of the Grand Canal south of Tientsin, where the 

 material was loess redistributed in the vast delta of the Hwang River, 

 but it was less compact. As the formation is homogeneous so far 

 as traced in the Barrancas del Norte, and as it is very similar to the 

 Pampean formations which extend from the Barrancas de los Lobos 

 for many scores of miles southward, we can not ascribe it to strictly 

 local conditions. On the contrary, it represents a general phase of 

 erosion and deposition which corresponds apparently to the removal 

 of fluvio-eolian formations from some one region and their redepo- 

 sition where they are now found. The writer is inclined to regard 

 this formation and similar deposits as due to river work on confluent 

 flood plains and consequently as made during a relatively humid 

 period. 



The upper surface of the basal formation in the Barrancas del Norte 

 is eroded and the hollows are filled by later deposits, sometimes of one 

 character, sometimes of another. Characteristic examples are rep- 

 resented in the illustrations (pis. 2, 3). It will be seen that the for- 

 mation was carved by an agent that undercut the sides and rounded 

 the bottoms of the hollows, leaving masses with sharp points or 

 edges in relief. Wind produces these effects in this material, whereas 

 water cuts channels having nearly vertical walls. Thus it would 

 appear that wind erosion, which is favored by, if not dependent on, 

 aridity, succeeded an epoch of deposition that was conditioned by 

 humidity. The eroded surface is not deeply carved but the extreme 

 relief of about two meters wliich it exhibits is probably near the 

 limit of height which the brown earth could maintain. It is impos- 

 sible to say how much may have been removed above this surface, 

 and we are thus left in doubt so far as this occurrence in the Bar- 

 rancas del Norte is concerned whether the erosion was local and tem- 

 porary or was occasioned by a general change. The phenomenon 

 recurs, however, in other exposures of the formation at Miramar and 

 at Necochea, and appears to be characteristic of the zone which 

 is now laid open along the coast. Thus it is not improbable that 

 the area of erosion was a broad one due to a somewhat general climatic 

 and geographic change. 



In the Barrancas del Norte the eroded surface of the Ensenadean 

 comes into contact with several formations, which are unlike in color 

 and constitution. The one which covers the longest stretches is a 

 greenish stratified deposit formed of Pampean earth, which has been 

 somewhat deoxidized. Similar greenish deposits occur here and there 



