Bowman — Geologic Relations of the Guzco Remains. 313 



the road seen in fig. 4. From the nature of the road-bed and 

 the bluff it is inferred that the bluff originally had a profile rep- 

 resented by the broken line of fig. 8 (left, bottom). The con- 

 struction of the road, therefore, further steepened a naturally 

 steep bluff and carried the face of it back far enough to expose 

 one end of a buried bone. It is possible that in building the 

 road other associated bones were excavated and lost ; also that 

 much more material of a similar nature might still be found. 

 Further excavation was impossible, however, because of other 

 problems and because of the desire to leave the bluff (at least 

 for the present) relatively undisturbed, so that interested stu- 

 dents might see it almost in its original condition. 



Physiographic Determinations. 



When the geologic history of comparatively fresh deposits 

 is desired, the application of physiographic principles is indis- 

 pensable. Fossils may be wholly absent, or if present may be 

 so closely related to the existing fauna and flora as to be of 

 little value. In the present case we have to determine, first, a 

 problem in structural geology — whether or not the bones 

 occurred interstratified in deposits in place; and, secondhand 

 more important, the age of the deposits as determined (1) by 

 the topographic forms developed on them and (2) by their geo- 

 graphic relations. The facts of structure are in this case rel- 

 atively simple. The physiographic facts and relations are 

 much more complex, though it is believed that they are no less 

 substantial and convincing. 



That the reader may have a proper guide in the examination 

 of the evidence, I shall at once present the conclusions that the 

 facts in the succeeding paragraphs seem to establish : 



1. The deposits in which the vertebrate remains were found 

 have the same age as deposits of similar composition and 

 topographic relations all about the borders of the Cuzco basin. 



2. The deposits belong to the glacial series; and to the 

 latter of two main groups. 



3. They were formed in a time of glaciation on the sur- 

 rounding highlands and of alluviation in the valleys. 



4. Since the burial of the Cuzco man thick bodies of gravel 

 were deposited, and later eroded. 



5. Since the deep erosion of the gravels there have been 

 two minor periods of alluviation. 



6. The gravels of glacial derivation rest upon (a) deformed 

 and eroded sedimentary rocks of Tertiary and Paleozoic age 

 and (b) igneous rock of pre-Tertiary age. 



The principal geologic and physiographic relations are shown 

 in fig. 10. The higher summits and intermediate slopes are 



Am. Jour. Sci.— Fourth Series, Vol. XXXIIT, No. 196.— April, 1912. 



21 



