Mbrriam.] 



John Day Basin. 



lake on a wide plain might by expansion and contraction, or shift- 

 ing its position, ultimately cover and bury bones which had been 

 resting for a long time on the land. The same thing could be 

 accomplished by flowing water in a basin where cutting had largely 

 ceased, or by accumulation of wind-carried dust or ash. 



The strongest argument in favor of the lacustrine hypothesis is 

 found in the character of the bedding of the series. The stratifica- 

 tion is nearly everywhere absolutely regular and the beds are fre- 

 quently very thin. Bedding of this kind could be developed only 

 on a floor free from all irregularities. Such deposits, where they 

 are not shown to have accumulated rapidly, are usually considered 

 as having been formed in water. 



Recently W. D. Matthew has suggested* that the White River 

 beds, which the John Day resembles in a measure, are in part of 

 aeolian origin. Except for the regularity and thinness of the 

 bedding, such an hypothesis as this applied to the question of the 

 origin of the John Day would have much in its favor. The 

 material being largely ash, there would be no difficulty in account- 

 ing for its origin. If the deposits were formed rapidly enough to 

 prevent dissection, absolute regularity of bedding could be main- 

 tained. Here again, however, we meet with a difficulty, as it is 

 hardly probable that a region subjected to frequent ash showers 

 would be inhabited by such numbers of large mammals as would 

 seem to have lived there. If the deposition took place at intervals 

 infrequent enough so that the region could be a desirable place of 

 habitation, it is probable that under ordinary conditions irregulari- 

 ties of the surface would cause noticeable unevenness in the bed- 

 ding. That the accumulation of the John Day beds occupied a 

 long period is brought out by a study of the vertical distribution of 

 the species, which changed considerably during this epoch. 



The thinness of the bedding in places might be urged as an 

 argument against a purely aeolian mode of deposition. The 

 writer is not familiar with any aeolian deposits that have accumu- 

 lated slowly, which show thin and well-marked bedding planes. 



A theory of fluviatile origin of the series would be open to 



*Am. Naturalist, May, 1899, Vol. 33, p. 403. 



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