188 G. K. GILBERT — CONTINENTAL PROBLEMS. 



inated on coasts. With the aid of such data are drawn the outlines of 

 ancient ocean and land at various geologic dates, and from the compari- 

 son of these outlines continental growth isinferred. In passing from the 

 formation boundaries of the geologic map to the oceanic limits of the 

 charts of ancient geography, allowance is made for the former extent of 

 non-littoral formations beyond their present boundaries. Tins allowance 

 is largely conjectural, and the range of possible error is confessedly great. 

 In passing from the observed limits of littoral formations to the coast 

 lines of ancient geography little or no allowance is usually made for the 

 former extent of the formations, and I conceive that great possibility of 

 error is also thus admitted. During a period of oceanic transgression 

 over the land all portions of the transgressed surface are successively 

 coastal, and the coastal deposits they receive are subsequently buried by 

 off-shore deposits. When, therefore, littoral beds are found in remnants 

 of strata surviving the processes of degradation, it is indeed proper to 

 infer the proximity of ancient coasts during their formation, but the in- 

 ference that they represent the limit of transgression for that epoch may 

 be far from the truth. For these reasons it appears to me that the spe- 

 cific conclusions which have been reached with reference to the original 

 extent of various formations are subject to wide uncertainties; and if 

 this be granted, then but brief attention to a simple law of denudation is 

 necessary to show that the general conclusion may be illusory. The 

 process of degradation by aqueous agencies is chiefly regulated, not by 

 the thickness of formations, hut by the height to which they are uplifted. 

 Thus the present extent of most formations is determined in large part 

 by crustal oscillations subsequent to their deposition. As formations are 

 progressively eroded, the underlying and older cannot be attacked until 

 the overlying and younger have been carried away, and so the outcrops 

 of the older of necessity project beyond the boundaries of the younger. 

 The process of vague inference, making indefinite allowance for the un- 

 known quantity of eroded strata, nearly always assigns to the older for- 

 mation, which projects visibly beyond the newer, a greater original 

 extent. It appears to me thus possible that the greater part of the data 

 from which continental growth is inferred maybe factitious and mislead- 

 ing. 



Furthermore, inference, such as it is, deals with only one phase of the 

 problem. It is applied to the incursions of the sea upon the land, but it 

 is not applied to the excursions of the land upon the sea. Just as we 

 infer from stratified rocks the presence of the sea, so also we infer from 

 unconformities the sea's absence; and to the student of ancient geography 

 the two classes of evidence are equally important. But the strata, spread 

 widely over the surface of the land, are conspicuous phenomena, while 



