40 BUREAU OF AMERICAN ETHNOLOGY [bull. 52 



to the methods that apply to more uniform marine strata, could 

 be successful. 



4. The criteria by which the several more or less theoretic 

 divisions of the Pampean have been distinguished are primarily 

 lithologic — composition, texture, compactness, structure, and the 

 proportion of secondary limestone or tosca. The differences in these 

 characters are not marked. At first the writer did not easily recog- 

 nize them. Eventually he saw them wdthout difficulty, but he did 

 not learn to rely on them as a means of classification. It seemed 

 to him that local conditions affecting the source of the material 

 and the character of the deposit might be more influential than 

 the greater or less age of the formation. It was apparent in clearlj^ 

 exposed sections that deposits of distinct epochs might occur in 

 the same plane in close proximity or in contact with one another. 

 Sections are few and the distances between them are great. Under 

 these conditions the subclassification of the Pampean is a problem 

 of extreme difficult}^, in which there is great liability to error. 



5. If we turn from the physical to the paleontologic criteria as a 

 means of classification, we are confronted with the fact that the 

 sequence of faunas can be determined with certainty only when 

 we know the sequence of formations. But, inasmuch as we do not 

 sureh' know the succession of Pampean formations, we can not have 

 confidence in any theoretic development of faunas. There is urgent 

 need in this matter to withhold judgment and to preserve a conserva- 

 tive attitude of agnosticism in regard to the relative age of hypo- 

 thetic divisions of the Pampean terrane and the supposedly distinct 

 faunas. 



6. Among tlie divisions of the Pampean which have been proposed 

 the distinction between the Upper Pampean and all older formations 

 of the terrane seems to be one of the safest. In many places there is 

 a distinct unconformity beneath the deposits of the Upper Pampean, 

 which is a result of uplift and erosion. Elsewhere there is entire 

 conformity, as, for instance, at San Lorenzo on the Parana, and 

 apparently continuous deposition from the older Pampean up into 

 loess deposits of the Upper Pampean type. This is not surprising, 

 for the surface of the pampas is now and has been in the past suffi- 

 ciently warped to bring' about conditions of erosion in one locality 

 concurrently with those of continuous deposition in another. The 

 Upper Pampean is identified lithologically as an eolian loess. It is 

 eolian not only in texture and structure but also in distribution, and 

 occurs generally in drifted deposits rather than as a widespread 

 stratum. The conditions for its formation may have been peculiarly 

 favorable during some recent epoch of aridity, and if so the formation 

 would acquire a certain unity and individuality. But the writer is 

 satisfied that not all of the irregularities of the surface which gave 



