OSCILLATORY CHARACTER OF CONTINENTAL SEAS 361 



black shale in eastern Oklahoma contains great, evidently ice-transported 

 boulders, and glaciated boulders are found in similar shales in Australia. 

 The Pottsville conglomerates, too, are highly suggestive of glacial drift. 

 Finally, as to the evidence furnished by the Pennsylvanian floras, this 

 seems to favor the newer interpretation quite as much as the old. Taking 

 the case as it stands it must be admitted that the Pennsylvanian black 

 shales, like those of earlier ages, are more readily explained on the 

 assumption of cool climates and shallow, perhaps swamplike, seas than 

 under the hypothesis of deposition at great depths in inclosed basins. 



Eegarding the suggested relation of black shale deposition to cool 

 climates it must be admitted that as a general explanation the propo- 

 sition is not altogether satisfactory. Probably the real cause, if there is 

 any that operated alike in all cases, remains to be discovered. So far 

 we have established only that none of the black shale deposits in America 

 is comparable in the matter of depth and inclosure of waters in which 

 they were laid down to the black muds in the Black Sea of today. Also 

 that there is no warrant for tlie uncjualified assertion that black shales 

 denote stagnant arms of the sea. 



SHALLOWNESS OF PALEOZOIC CONTINENTAL SEAS AND ABSENCE OF 

 STRONG, TRANSCONTINENTAL CURRENTS 



Continental seas generally very shallow. — The few selected facts which 

 have been discussed on preceding pages, and to which is to be added 

 much similar evidence incidentally brought out later on, will probably 

 suffice to establish the average exceedingly low relief of Paleozoic lands. 

 It is reasonable to infer that the continental seas were correspondingly 

 shallow. Direct evidence on depth of waters is afforded by the Eich- 

 mondian and Niagaran embayments of the Nashville and Ozark uplifts. 

 Granting, further, what I believe all will admit, tliat the present depth 

 of simple downwarps is probably greater, and can not be less, than it 

 was at the time of their occupancy by arms of Paleozoic seas, the facts 

 observed in something like 20 instances of such embayments indicate be- 

 yond question that the maximum depth of their waters at no time ex- 

 ceeded 100 feet and generally must have been much less. As regards 

 depth of the broader interior seas direct evidence concerning the older 

 Paleozoic is almost wanting. Subsequent and repeated accentuations of 

 the originally doubtless very gentle warps have deeply buried the compe- 

 tent factors. Judging from the present altitude and slow rate of over- 

 lap of the Kinderhook, Chester, and lower Pottsville deposits, which lie 

 on the south flank of the Ozark dome and certainly failed to extend over 

 XXV— Bull. Geol. Soc. Am., Vol. 22, 1910 



