DISCUSSION OF EVIDENX'E 165 



to an elevation of 500 feet above tide, but never above this level, even 

 where the silts are well exposed from their top to the bed rock, would 

 seem to point to water as an important, if not the controlling, factor in 

 the deposition of the stratified beds. 



Of the fossils all but those from near Mounts are of land species. This 

 presents at first sight a strong argument that the silts which contain them 

 are of land origin. It was early noted, however, that the fossils are not 

 found in curved layers or pockets as if they had accumulated in depres- 

 sions of the land surface, but are generally found distributed through the 

 minute laminse of the silts. The perfection of these laminae show conclu- 

 sively that they have never been penetrated either during the period of 

 their accumulation or at any time since by rootlets of any sort, even by 

 the minute rootlets of the grasses. If the fossils are to be considered as 

 being indigenous to the deposit, which is thus assumed to be a land ac- 

 cumulation, we should have the anomalous state of affairs where an 

 abundant fauna, consisting of species which are characteristic of wet 

 wooded situations, was living where there was a complete absence of veg- 

 etation, and where consequently there was no food. Such an absence of 

 vegetation could only occur where the climate was either too cold or too 

 dry for vegetation to live, neither of which conditions, it is practically 

 certain, existed in this region during the period of marl-loess accumula- 

 tion. On the other hand, it seems only natural that land forms should 

 be washed or floated into a body of fluctuating water, and deposited 

 with its sediments, while a general absence of aquatic species may be 

 explained as resulting from a high silt constituent of the waters. 



It has sometimes been urged that the preservation of certain of the 

 delicate and fragile forms indicates that water has played no part in the 

 formation of the deposits, it being regarded as too violent in its action. 

 Any one who has seen the relatively heavy shells of some of she smaller 

 littoral shells taken up and whirled along by the winds on our coasts, 

 and who has compared the violence of this action with that of a sluggish 

 silt-depositing stream, will have no doubt as to which is the best calcu- 

 lated to preserve the delicate shells. The presence of the operculum 

 within the opening simply means the shell was buried by succeeding 

 deposits before decay of the animal matter had set it free, and the lack 

 of filling of the whorls, which is exhibited by a relatively small number 

 of the shells, indicates nothing except that in these special instances 

 there was no prolonged rolling or similar movement of the shells by 

 means of which the silt worked its way back into the shell. The fossil 

 shells are not always complete, in fact some laminse and layers seem to 

 be made up largely of the comminuted fragments of shells. This, how- 

 ever, though sometimes considered as evidence of aqueous origin, could, 



