CHAPTER VI. 



THE RELATION OF ALLUVIAL TERRACES TO MAN. 



From the purely physical phenomena of fluvial terraces, lacustrine strands, and sand- 

 dunes we have been led to a broad generalization as to the pulsatory nature, decreasing 

 intensity, and present continuance of post-glacial changes of climate in the arid portions 

 of America. From the same lines of evidence a similar conclusion has been reached as 

 to the temperate portions of Asia and the lands surrounding the Mediterranean Sea. In 

 the Old World, however, it has been possible to supplement physical evidence by a large 

 body of historical and archeological data, together with traditions and legends, and thus 

 to determine the approximate dates of some of the main climatic events and to discover 

 some of their effects upon man. In general we are probably safe in assuming that the 

 effect of a given type of change upon man will be essentially the same in corresponding 

 parts of the two hemispheres, but this needs careful testing. The Asiatic dates, on the 

 other hand, do not necessarily afford any clue whatever to the dates of similar climatic 

 changes in America. Hence our next step must be to find out the relation of man to the 

 changes of cHmate whose existence we have inferred in America and then to determine the 

 dates. It must be constantly borne in mind that the belief that changes have taken place 

 at certain times and in certain ways in the Old World by no means involves a similar beUef 

 in respect to the New World. Three possibilities present themselves. In the first place, 

 granting that pulsatorj' chmatic changes have taken place in both hemispheres, it is 

 possible that those in America came to an end long before those in Asia and thus had no 

 influence either upon the Europeans who came in the wake of Columbus or upon the ancient 

 inhabitants who preceded them. In the second place, changes may have taken place in 

 the climate of both hemispheres similar in kind, but by no means synchronously. They 

 may even have been of opposite types, America becoming dry when Asia became moist, 

 and the reverse. Finally, there is the third possibility that all the continents, or at least all 

 the temperate regions of the northern hemisphere, have been subject to the same type of 

 changes at essentially the same times. Leaving the matter of dates for futm-e consider- 

 ation, let us attempt to determine whether any one or more of the changes of chmate which 

 we have inferred to have taken place in America occurred since man reached a stage of 

 culture such that he inhabited permanent villages or towns whose traces still remain. 

 The importance of this subject is so great that we shall investigate it at length, and shall 

 in many cases enter into minute details of evidence in widely scattered regions. I shall 

 endeavor to set forth the facts in such a way that the reader can frame his own answers 

 to three chief questions. First, has the climate of America changed since the primitive 

 inhabitants built the ruins which now abound in the Southwest and Mexico? Second, if 

 it has changed, do the ruins show evidence of changes in more than one direction or at more 

 than one distinct epoch? And third, do the inferred changes seem to have had effects at 

 all comparable to those which seem to have taken place in Asia? 



Let us first take up the specific problem of the relation of man to the alluvial terraces 

 which hold so important a place among the purely physical evidences of pulsations. If we 

 accept the theory of the chmatic as opposed to the tectonic origin of the terraces of the 

 Southwest, the finding of pottery or other traces of human occupation, either within the 

 body of a terrace or persistently upon some terraces and not upon others, may be sig- 

 nificant. In regard to the second point, the finding of traces of human occupation on some 



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