November 17, 1893.] 



SCIENCE. 



277 



it had done so, that so little advance in denudation of the 

 uplifted mass should have as yet been accomplished. 

 There is every indication that before the close of Pale- 

 ozoic time, the region which we now call Missouri suffered 

 many oscillatians of level, for its strata are varied in com- 

 position and are separated by slight unconformities. The 

 unconformable superposition of the Cretaceous and Ter- 

 tiary strata of the Jlississippi embayment on the denuded 

 surface of the deformed Paleozoic rocks indicates that 

 changes of level and warpings occurred during and after 

 Mesozoic time not far from the region under considera- 

 tion. In the absence of direct evidence of actual stability, 

 moderate oscillations of level through vertical distances 

 of a few hundred feet, or perhaps as much as a thousand 

 feet, with slight warpings involving slants of a degree or 

 two, should, I think, be regarded as characterizing the 

 post-Paleozoic history of Missouri, as well as its Paleo- 

 zoic history. 



Just when the post-Paleozoic oscillations occurred, and 

 just -what was their amount is not determinate; but the 

 latest important oscillation of the series is the one to 

 which I would refer the permission of the rivers to cut 

 their present deep valleys. The various brief and subor- 

 dinate oscillations associated with glacial invasions and 

 deposits of loess are comislicated beyond clear under- 

 standing at my distance from their local evidence. 



But oscillations being neglected, if Missouri had had 

 only one cycle of denudation since its uplift at the end 

 of the Paleozoic, it could not still be a high- 

 land. If the present altitude of the Ozark uplift 

 with respect to its surroundings had been taken at 

 the end of Paleozoic time, as Mr. Winslow supposes, why is 

 it not all consumed now ? The sufficiency of subaerial 

 erosion to reduce great ujilifts to lowlands in less than 

 the whole of post-Paleozoic time is, I believe, well demon- 

 strated. I do not mean to say that this demonstration is 

 generally accepted ; for curiously enough, there is a pre- 

 vailing hesitation of belief on this subject. Geologists 

 have not as a rule given the matter much attention, but 

 this does not weaken the validity of its demonstration. 

 Those who have carefully looked into the matter, are, I 

 think, convinced of its correctness. Others with whom I 

 have talked on this question, having their own special 

 studies in other directions, have expressed a general in- 

 credulity about it, doubting that Mesozoic time was long 

 enough to wear down mountains to penejalains; but their 

 doubts have not taken the form of effective argument. 

 Such doubts might have more value if we had not in 

 many well-known deposits of stratified rocks, the direct 

 evidence of the sufficiency of erosive forces to accomplish 

 great res alts within definitely limited divisions of the 

 geological time scale; and if we had not sufficient studies of 

 land forms to show that even a part of post-Paleozoic 

 time is long enough to baselevel uplifted masses. 

 Referring only to examples with which I am personally 

 familiar, I may mention the following districts as instances 

 of effective base-levelling within determinate geological 

 periods: 



The plains of the upper Missouri, about Fort Benton, 

 Montana, consist of Cretaceous strata, having a broadly 

 rolling surface of slight relief over large distances; but 

 here and there, surmounted by lava-capped mesas, or by 

 necks and thick dikes of lava, whose present position can 

 only be explained by supposing that the strata of the sur- 

 rounding plains once rose at least as high as, if not higher 

 than, these eminences. Yet this greater mass is now re- 

 duced to a peneplain; and since its reduction to a pene- 

 plain, it has been uplifted bj' a considerable amount, and 

 the present trench-like valleys of the Missouri and its 

 branches have been cut down two or three hundred feet. 



All this has happened since the deposition of the Creta- 

 ceous strata, of which the plains are there built. It is 

 true that the strata of the plains are not particularly 

 resistant; but neither are those of the Missouri plateau. 



The Triassic formation of Connecticut and New Jersey 

 has been base-levelled since it was faulted and tilted from 

 its original horizontal jjosition. Since it was base-levelled 

 the resultant peneplain has been again uplifted, and its 

 sandstones have been reduced to a second base-level, 

 while its very resistant trap rocks retain, more or less per- 

 fectly, in their crest lines an indication of the altitude to 

 which the older peneplain was elevated. The first work 

 of denudation, by which even the traj) sheets and the ad- 

 jacent crystalline rocks were effectively base-levelled, was 

 a j)ost-Triassic work; the second denudation, by which 

 only the weaker sandstones were base-levelled, is roughly 

 dated as post-Cretaceous. The base-levelled sandstones 

 are now trenched, in consequence of a late, or post-Tertiary, 

 uplift. 



In Pennsylvania the mountain ridges that are generally 

 described as the remnants of the Appalachian or post- 

 Carboniferous folding and uplift, cannot be legitimately 

 so considered in the light of existing evidence. Their 

 extraordinarily even crest lines, entirely out of accord 

 with their folded structure, but closely in accord with 

 one another, can be interpreted only as surTiving indica- 

 tions of the peneplain to which the whole mountain sys- 

 tem was reduced while the region stood lower than it 

 now does; and the wide open valley lowlands between the 

 ridges are the product of denudation since the uplift of 

 the peneplain. These valley lowlands are trenched by 

 the streams, in consequence of a still later uplift. The 

 dates of these features are apparently identical with 

 their relatives across the Delaware in New Jersey. 



The upland of the Appalachian plateau in western 

 Pennsylvania is a surface of denudation, trenched by val- 

 leys. The upland is accordant in altitude with the even 

 crest lines of the Appalachian ridges. 



The Hudson Eiver flows through its crystalline High- 

 lands in a deep, steei^-sided valley. Further up stream, 

 above Newburgh, where the rocks are weaker, the valley 

 is opened into a broad lowland. Both the gorge of the 

 Highlands and the open valley lowland further up stream 

 are the work of post-Cretaceous erosion, and jjrobably of 

 less than all of Tertiary time. The valley lowland is 

 trenched, indicating a late Tertiary or a post-Tertiary 

 uplift. 



Examples of this kind might be increased in number 

 from the western surveys, but I shall leave observers 

 there to speak for themselves. They all teach one lesson, 

 namely, that in rocks of moderate hardness Tertiary time 

 was amply long enough to allow the formation of wide 

 open valleys, even to produce peneplains of faint relief 

 on such rocks as the Triassic sandstones of New Jersey, 

 the Paleozoic shales and limestones of Pennsylvania, or of 

 the middle Hudson valley. It was long enough to form 

 narrow valleys in rocks of excessive resistance, like those 

 of the Hudson Highlands. 



Is not this conclusion applicable to Missouri? The 

 rocks along the Osage are not of notable resistance. How, 

 then, can its valley slopes be stee^j if thej' are so old as 

 all of Mesozoic and Tertiary time! That measure of time 

 has elsewhere easilj' sufficed to wear out highlands into 

 lowlands, to uf)lift them again, and enter well upon their 

 second effacement. How, then, can Missouri be still so 

 little advanced in the sculpturing of its topography, ex- 

 cept by reason of the relatively recent renewing of the 

 task! It seems to me utterly imi^ossible to explain the 

 valleys of Missouri as a product of one geographical 

 cycle; the product of sculpturing that has been "uninter- 



