April 27, 1SS3.] 



SCIENCE. 



325 



eua. The decaj' of the primal and auroral 

 strata of the Appalachian valley, and the 

 formation therein of cla3's and of iron and 

 manganese oxides, was also discussed. The 

 pre-Cambrian antiquity of the process of decaj- 

 in the eozoic rocks of the Mississippi valley, 

 as shown by Pumpelly and by Irving, as well 

 as similar evidence from Europe, was noted, 

 wliile the more recent decomposition seen in 

 the auriferous gravels of California was de- 

 scribed and explained. 



The final removal of the covering of decaj'ed 

 rock from man3' northern regions during the 

 drift period was then considered ; and the thesis 

 advanced by the speaker in 1873, that the decay 

 of rocks ''is an indispensable preliminarj' to 

 glacial and erosive action, which removed pre- 

 viously- softened materials," was discussed in 

 its relations to bowlders, glacial drift, and the 

 contour of glaciated regions. Pumpelly's de- 

 velopment and extension of this doctrine to 

 wind-erosion was noticed, and also the recent 

 comparative studies of Reusch in Norway and 

 in Corsica, in which similar views are enforced. 



The principal points in the paper, as re- 

 viewed at its close, are as follows : — 



1. The evidence afforded by recent geologi- 

 cal studies in America and elsewhere, of the 

 universality and the antiquity of the suhaerial 

 decaj", both of crj'stalline silicated rocks and 

 of calcareous rocks, and of its great extent in 

 pre-Cambriau times. 



2. Tlie fact that the materials resulting 

 from such decaj" are preserved in situ, in regions 

 where thej' have been protected from denuda- 

 tion by overlying strata, alike of Cambrian 

 and of more recent periods ; or, in the absence 

 of these, by the position of the decayed rock 

 with reference to denuding agents, as in drift- 

 less regions, or in places sheltered from erosion, 

 as within the .St. Lawrence and Appalachian 

 valleys. 



3. That this process of decay, though con- 

 tinuous through later geological ages, has, 

 under ordinary conditions, been insignificant 

 ill amount since the glacial period, for the rea- 

 son that the time which has since elapsed is 

 small wlien compared with previous periods ; 

 and also, probablj', on account of changed at- 

 mospheric conditions in the later time. 



4. That this process of decay has furnished 

 the material, not only for the clays, sands, and 

 iron oxides from the beginning of paleozoic 

 time to the present, but also for the corre- 

 sponding rocks of eozoic time, which have been 

 formed from the older rocks by the more or 

 less complete loss of protoxide bases. The 

 bases thus separated from crystalline silicated 



rocks have been the source, directly or indi- 

 rectly, of all limestones and carbonated rocks, 

 and have, moreover, caused profound , secular 

 changes in the composition of the ocean's water. 

 The decomposition of sulphuretted ores in the 

 eozoic rocks has given rise to oxidized iron 

 ores in situ, and to rich copper deposits in vari- 

 ous geological periods. 



5. That the rounded masses of crj'stalline 

 rocks, left in the process of decaj", constitute 

 not onh^ the bowlders of the drift, but, judging 

 from analogy, the similar masses in conglom- 

 erates of various ages, going back to eozoic 

 times ; and that not only the forms of such 

 detached masses, but the surface-outhnes of 

 eroded regions of crj'stalline rocks, were deter- 

 mined by the preceding process of subaerial 

 deca}' of these rocks. 



THE ORIGIN OF CROSS-VALLEYS. 

 I. 



Dr. Fr. Lowl of Prague contributes an in- 

 teresting article on Die JEntstehung der DurcJi- 

 bruchsthdler to a recent number of Peter- 

 niann's mittheilungen (1882, 405—1:16), -and 

 comes to the conclusion that transverse vallej'S 

 or water-gaps are never formed bj' the per- 

 severing action of an antecedent or pre-exist- 

 ing river on a slowlj' rising mountain fold or 

 fault. " Erosion can, under no circumstances, 

 keep pace with mountain folding" (409). 

 Cross-valleys are then accounted for in two 

 other ways, — first, occasionally by erosion at 

 the outlet or point of overflow of the lake 

 formed behind the rising mountain barrier ; 

 second, and so frequently as to constitute the 

 general method, by backward erosion at the 

 head of a lateral valley, which finally cuts 

 through the ridge separating two longitudinal 

 valleys, and allows the higher to drain across 

 into the lower, so that in a folded mountain 

 system of great age the original order of 

 drainage on the longitudinal valleys is often 

 entirely effaced (411). Several carefully ex- 

 amined cases of this kind are described for 

 the eastern Alps and elsewhere. The ques- 

 tion does not arise now whether these exam- 

 ples are correctly determined : presumably 

 those to which sufficient local stud}' was given 

 are decided safeh' enough ; for this backward 

 origin of certain gorges is eminently possiljle. 

 The question is rather, whether nearlj' all cross- 

 valleys ai'e of this ancestry, and whether the 

 antecedent valley nowhere exists. We con- 

 sider Lowl's affirmative answer to this ques- 

 tion essentiall}- incorrect, and believe that his 



