644 REPORT — 1897. 



IV. The drainage of both the Arkansas and Texas Carboniferous areas was 

 reversed about the end of Jurassic times, when orographic movements over South- 

 east Arkansas, Eastern Texas, Louisiana, and Mississippi submerp-ed the former 

 extension of the Appalachian watershed, and admitted the early Cretaceous sea 

 across the Palaeozoic land as far north as Southern Illinois. 



V. This depression was not a deep one (Hilgard^) and did not all occur at one 

 time, for there have been subsequent distiu'bances of a more or less similar nature 

 in the same region. 



VI. The evidences of this depressions are — 



1. The reversed drainage of the Arkansas vallej'. 



2. The reversed drainage over the Carboniferous area of Central Texas. 



3. The submerged eastern end of the Ouachita uplift of Arkansas. 



4. The eastern slope of the peneplain of the Ouachita region. 



5. The direction of the faults and folds near the eastern exposure of the Lower 

 Coal Measures in Arkansas. 



6. The great fault thi-ough Texas near the Tertiary border having a down-throw 

 of 1,000 to 1,500 feet on the south and east side. 



7. Eruptive rocks accompanying the Texas fault and the Tertiary border 

 through that State and Arkansas to the Arkansas River. 



8. Hot springs near the same line. 



9. Faults in Alabama with a down-throw of 10,000 feet or more on the north- 

 west side. 



10. The thickness of the Cretaceous and Tertiary sediments over the depressed 

 area— from 4,000 to 10,000 feet. 



VII. The south-western or Central Texas end of the Appalachian land areas was 

 formerly covered by Cretaceous sediments, but it has since been uncovered by 

 erosion ; further east it is still concealed. 



VIII. The Carboniferous beds uncovered in Texas all belong to the Upper 

 Coal Measures, except at the edge of the synclinal trough ; it is inferred that a 

 greater thickness is still covered. 



IX. Tlie character of both the Silurian and Lower Coal Measures sediments of 

 the Ouachita uplift show that they came from the south, so that the land area 

 must have been in that direction during PalaBOZoic times. 



X. The sea occasionally invaded both the Arkansas and Texas synclinal troughs 

 during Coal Measures times, but coal-forming conditions obtained in the Texas 

 eyncline later than in Ihe Arkansas basin. 



XI. The Tertiary depression was probably more marked on the Arkansas than 

 on the Tennessee side of the embayment : this is suggested by the Cretaceous 

 border being concealed by thin Tertiary deposits in Arkansas, while in Tennessee, 

 Mississippi, and Alabama it is exposed in a broad belt. 



7. Report on the Investigation of a Coral Reef. See Reports, p. 297. 



FRIDA Y, A UG UST 20. 

 The following Papers were read : — 



1 . A Group of Hypotheses hearing on Climatic Changes?- 

 By T. C. Chamberlix, Professor of Geology in the University of Chicago. 



A computation of the several constituents of the atmosphere and of the rate at 

 which they are being consumed in the alteration of the surface rocks indicates that 

 in a comparatively few thousand years the carbonic acid of the air will be exhausted 

 if there is no compensating source of re-supply. The ocean contains about 18 times 



' Amer. Jour. Sci., 1874, vol. cii. p. 394. ' jg^^^ ^f Qggi (Chicago), vol. v. p. 653. 



