458 



SCIENCE 



[N. S. Vol. LII. No. 1350 



the members of the Egyptian Geological Sur- 

 vey " became obsessed by the notion that the 

 ISTile Valley and the Gulf of Suez were parts 

 of the rift system," and that a later interpre- 

 tation of both these features as valleys of 

 erosion — one of them moderately submerged 

 — was much retarded by that premature con- 

 clusion. He rejects Gregory's correlation of 

 the Pacific basin and the African plateau, 

 doubts the production of rift valleys by 

 tension, suggests that their structure may be 

 due to compression, and notes that their form 

 may have been much influenced by erosion, 

 especially in those African depressions which 

 have a scarp only on one side. He therefore 

 wisely urges that the whole problem should 

 be regarded as needing much further investi- 

 gation before a safe conclusion can be 

 announced. 



Ball is, however, over-conservative in assert- 

 ing that " the only sure proof of the existence 

 of a trough fault is to be obtained by tracing 

 across the floor of the trough the same strata 

 ■as occur at higher levels in the boimding 

 scarps." This overlooks the competent phys- 

 iographic evidence of faulting that is pro- 

 vided when a nearly rectilinear scarp truncates 

 a series of deformed strata or a body of 

 massive rocks, as pointed out by Gilbert in 

 his studies of the Great Basin ranges. For 

 example, the trough of the Rhine from Basal 

 to Bingen rarely shows rock outcrops on its 

 floor; evidence of its depression between sub- 

 parallel faults is derived chiefly from the 

 topography of its enclosing scarps. Simi- 

 larly, the Limagne, a broad rift-depression 

 in central France, drained northward by the 

 AJlier between the highlands of Auvergne on 

 the west and those of the Monts du Forez on 

 the east, is floored with lacustrine or fluviatile 

 sediments of modern date, whil§ the enclosing 

 highlands consist of ancient crystalline rocks. 

 The existence of marginal faults here has long 

 been accepted by French geologists, although 

 the kind of evidence dmanded for faults by 

 Ball is not forthcoming. Crystalline rocks 

 are not seen on the floor of the depression, 

 and even if they were, they could not be 

 proved to be down-faxilted. The evidence of 

 down-faulting is found in the nature of the 



enclosing scarps. It may be noted in pass- 

 ing that the Limagne is not strictly a rift 

 valley, but a resequent rift valley, in. that 

 since its first production by down-faulting, 

 when it was truly a rift valley, it has been 

 filled with inwashed sediments to the level 

 of the enclosing highlands at a time when the 

 whole region stood lower than now, and later 

 on, after a broad elevation without faulting, 

 the inwashed sediments have been washed 

 out as deep as river-grade permits, thus again 

 leaving the enclosing scarps in relief. The 

 evidence of this succession of events is clearly 

 fuVnished by the presence of isolated volcanic 

 necks in the midst of the depression, and of 

 surviving spurs of the weak sediments capped 

 with lava flows, that project into the depres- 

 sion about at the level of the adjacent high- 

 lands. A resequent rift-valley of this kind 

 must evidently differ from an initial rift- 

 valley in having the height of its enclosing 

 scarps determined by the depth of recent 

 erosion, not by the drop of the original 

 faulting. 



To return to Africa: If so great a series of 

 rift valleys really exists there as is represented 

 on Gregory's map, some of them should show 

 scarps that truncate the structures of the 

 enclosing higUands, and the evidence that 

 such scarps provide for down-faulting should 

 not be overlooked. The possibility that some 

 of the African rift valleys have been filled and 

 excavated again in resequent fashion like the 

 Limagne should also be inquired into. 



W. M. Davis 



CAMBRniGE, Mass., 

 October 31, 1920 



SCIENTIFIC EVENTS 



THE SIXTEENTH ANNUAL NEW ENGLAND 



INTERCOLLEGIATE GEOLOGICAL 



EXCURSION 



The sixteenth excursion of the New Eng- 

 land geologists was taken in the vicinity of 

 Middletown, Connecticut, October 8 and 9, 

 under the direction of Professors William 

 North Rice and Wilbur G. Foye. About 

 twenty-five persons were in attendance, among 

 whom were representatives from Harvard, 

 Massachusetts Agricultural College, Moiuit 



