CLIMATE AND TERRESTRIAL DEPOSITS 377 



change which is known to have taken place at about that time seems 

 a sufficient hypothesis, and renders the other necessary only where 

 warping or tilting of previous base levels can be demonstrated. In 

 graded rivers of considerable volume a slight increase of volume, by 

 producing erosion along the whole stream course would seem compe- 

 tent to sweep sand and gravel to great distances, which previously 

 could not be transported along the bottom, resulting in the deposit 

 of hundreds of feet of gravels at New Orleans whither now the 

 Mississippi carries chiefly clay and in its channel nothing coarser 

 than sand."^ 



On the other hand, W. J. McGee points out that the Lafayette 

 bordering the continent was laid down during an epoch of land 

 subsidence and that these Quaternary gravels may therefore be 

 reworked Lafayette gravels, owing to a Quaternary land elevation 

 and rapid erosion in the lower portions of the rivers.^ To account 

 for the original Lafayette gravels of the lower Mississippi, carried 

 seaward during a low stand of the land, a hypothesis of climatic 

 change appears to the present writer to apply most readily. If the 

 Quaternary gravels consist, however, of a redeposition during a low 

 stand of the sea and not during a local and progressive subsidence 

 of the lower Mississippi, then such a fluctuating sea level is sufficient 

 to account for a Quaternary redeposition without invoking as a 

 further aid a climatic cause, even if such may have been also in 

 operation. 



Irrespective as to whether or not, however, the orange sand of the 

 Mississippi Valley is the formation which represents in part the prod- 



I Since the above was written two further communications have appeared bearing 

 upon the age and mode of origin of the "Lafayette beds" of Louisiana. G. D. Harris 

 records the existence of sands, clays, and gravels, the latter of typical Lafayette type, 

 extending to a depth of at least 1,500 feet; while a Quaternary molluscan fauna extends 

 down to about 2,000 feet. The well records thus seem to indicate that the seaward 

 continuation of the gravels in the central portion of Louisiana as well as in those 

 states to the east and west are rather Quaternary than Pliocene. It would seem, then, 

 that Hilgard's views as to the contemporaneousness and interrelationship of the coarse 

 "Orange sands" in the south and the ice sheets in the north may prove correct in 

 spite of the fact that certain "Lafayette" gravels are said to lie beneath glacial till 

 farther north. (G. D. Harris, "Note on the 'Lafayette Beds' of Louisiana," Science, 

 N. S., Vol. XXVn, 1908, p. 351.) ' • 



=> Science, N. S., Vol. XXVII, 1908, p. 472. 



