194 THE ANDES OF SOUTHERN PERU 



ward, sedimentation may be carried on continuously and a clear 

 geologic record may be made. 



2. Even if crustal movements are alternately downward and 

 upward, satisfactory conclusions may be drawn from both (a) the 

 nature of the buried surfaces of erosion, and (b) the alternating 

 character of the sediments. 



3. If, however, the deformative processes effect steady or in- 

 termittent uplifts, there may be no sediments, at least within the 

 limits of the positive crustal units, and a geologic record must be 

 derived not from sedimentary deposits but from topographic 

 forms. We speak of the lost intervals represented by strati- 

 graphic breaks or unconformities and commonly emphasize our 

 ignorance concerning them. The longest, and, from the human 

 standpoint, the most important, break in the sedimentary record 

 is that of the present wherever degradation is the predominant 

 physiographic process. Unlike the others the lost interval of the 

 present is not lost, if we may so put it, but is in our possession, 

 and may be definitely described as a concrete thing. It is the 

 physiography of today. 



Even where long-buried surfaces of erosion are exposed to 

 view, as in northern Wisconsin, where the Pre-Cambrian paleo- 

 plain projects from beneath the Paleozoic sediments, or, as in New 

 Jersey and southeastern Pennsylvania, where the surface devel- 

 oped on the crystalline rocks became by depression the floor of the 

 Triassic and by more recent uplift and erosion has been exposed 

 to view, — even in such cases the exposures are of small extent and 

 give us at best but meager records. In short, many of the breaks 

 in the geologic record are of such long duration as to make im- 

 perative the use of physiographic principles and methods. The 

 great Appalachian System of eastern North America has been a 

 land area practically since the end of the Paleozoic. In the Cen- 

 tral Andes the "lost interval," from the standpoint of the sedi- 

 mentary record, dates from the close of the Cretaceous, except in 

 a few local intermont basins partially filled with Tertiary or 

 Pleistocene deposits. Physiographic interpretations, therefore, 

 serve the double purpose of supplying a part of the geologic rec- 



