704 



SCIENCE 



[N. S. Vol. XXV. No. 644 



terms. However, the smootliness of the 

 Lafayette plain, where it is still preserved, 

 taken with the moderate thicknras of its de- 

 posits (50 or 100 feet), would indicate that 

 great advance had been made toward base- 

 leveling by pre-Lafayette subaerial erosion; 

 and that whatever residual relief then sur- 

 vived on the Cretaceous-Tertiary coastal plain 

 area was effectually abraded by the advancing 

 Lafayette sea. This conclusion evidently 

 postulates the marine origin of the Lafayette, 

 an origin generally accepted and not here 

 brought into question. 



2. The stage of erosion reached in the 

 Lafayette- Sunderland interval of uplift. A 

 comparison of statements in different chapters 

 of the report and an examination of the rela- 

 tion between the several terrace scarps and the 

 main valleys lead the reviewer to infer that 

 the uplift of the renovated, or Lafayette- 

 covered coastal plain was sufficient in amount 

 and duration to allow its erosion by normal, 

 river-led processes in the Lafayette-Sunder- 

 land interval to a stage of advanced maturity. 

 The valleys thus formed seem to have com- 

 pared well in width and depth, as stated above, 

 with those of the Potomac and Chesapeake of 

 the present day. The text of the report is, how- 

 ever, not immediately clear on this important 

 point; and this is the more to be regretted, 

 because a definite physiographic picture of 

 the district at the beginning of the strong de- 

 pression which culminated in the erosion of 

 the Sunderland scarp is an essential basis for 

 a clear understanding of the development of 

 the several terraces. The difficulty here seems 

 to be that the problem of the Lafayette- 

 Sunderland interval is not completely stated 

 on an early page, and then referred to, when 

 occasion again arises, in the terms first 

 adopted for its description; but that it is 

 stated partially in different chapters, in vari- 

 ous connections, and in diverse terminology, 

 through all of which the reader must pursue 

 his inquiry before he can acquire the writer's 

 point of view. For example, after reading (as 

 quoted above) that " there was a long interval 

 of erosion before deposition of the Lafayette 

 beds began," the following lines state that 

 ' another long period of erosion occurred ' 



before the Sunderland terrace was formed 

 (p. 78). This would naturally give the im- 

 pression that the Lafayette-Sunderland ero- 

 sion interval was of great importance, for the 

 pre-Lafayette erosion went far towards base- 

 leveling the coastal plain of earlier uplift. 

 On the other hand, in the final summary, the 

 ' elevation and erosion ' between the Lafayette 

 and Sunderland formations is stated in pre- 

 cisely the same terms as the ' elevation and 

 erosion ' between each pair of the succeeding 

 formations (p. 137) ; and inasmuch as the 

 later erosion intervals are well proved to have 

 been of moderate duration (as will be shown 

 below), this would suggest that the Lafayette- 

 Sunderland interval was also of no great 

 length. The reader who glances, as some 

 readers may, over the closing summary of the 

 report before reading the details of the preced- 

 ing pages, will naturally gain from the sum- 

 mary an impression that all the post- 

 Lafayette erosion intervals were of about the 

 same length. But this impression would have 

 to be changed on further reading. For ex- 

 ample, it is btated on an earlier page : " The 

 salient features of the Coastal Plain topog- 

 raphy were outlined at this time [Lafayette- 

 Sunderland interval], although it is doubtful 

 if they received their full strength or final 

 touches before the post-Talbot uplift " (p. 

 123). This would indicate that the interval 

 imder discussion was long enough for the 

 attainment of a mature stage of erosion; yet 

 the next earlier sentence states : " After the 

 deposition of the Lafayette formation, the 

 land was raised above ocean-level and sub- 

 jected to an interval of erosion which was 

 probably of longer duration than the later 

 ones which separated the other surficial de- 

 posits [terraces] of the series " (p. 123). 

 Here the phrase, ' probably of longer dura- 

 tion ' destroys all the emphasis that might be 

 given to the duration of the Lafayette-Sunder- 

 land interval by the previous citation. Again, 

 the statement that the Sunderland formation 

 " extends up into ancient valleys which pene- 

 trate it [the Lafayette] as reentrants " (p. 

 87), coupled with the fact that these re- 

 entrants are shown by the general map to 

 stand on the opposite sides of larger vaUeys, 



