Shaler.] 416 [Dec. 7, 



Where a series of limestones, originally in the bedded form, has 

 been extensively metamorphosed, the process of change often brings 

 about a consolidation of the whole mass. The clay is infiltered 

 with lime to such an extent that the beds of it appear only as dis- 

 colored portions of the whole section and not as distinct partitions. 

 It is not to be denied that there are other possible actions which 

 may serve to produce partitions in the rocks, but the one above 

 suggested seems to be the most likely cause which is known to us 

 to be of an efficient nature. 



In our ordinary limestones of paleozoic age the average number 

 of successive limy layers and shales may be counted as about 

 four to each foot in depth or in such a succession as the Trenton 

 limestone which may be called fifteen hundred feet in thickness 

 there may be as many as six thousand of these divisions. It seems 

 not unreasonable to suppose that in a period which continued as 

 long as the Trenton this number of earthquake shocks may have 

 affected an average sea bottom. Moreover this hypothesis will 

 account for the interesting though little observed fact that the 

 number of these divisions in a given thickness of strata varies 

 greatly in different portions of the world. Thus we may find in 

 one region the divisions of the strata amounting to as many as six 

 to the foot, while in another region a thousand miles away the 

 divisions may not exceed one to the foot of depth. From all we 

 know of earthquake action we may fairly presume that the lia- 

 bility to shocks in particular regions has varied greatly and thus 

 a variation in number of the partition planes would be brought 

 about. It may indeed be the fact that in certain regions a geo- 

 logical period may go by without any shocks of decided violence 

 on the sea floor. Thus in the North Atlantic there seems to be in 

 the present period a notable exemption from submarine disturb- 

 ances of a seismic nature. Some recent studies which I have 

 made on the kame deposits along our American shore clearly in- 

 dicate that no inundating waves such as are produced by earth- 

 quakes have rolled upon the coast of New England since the shore 

 assumed its present level. We cannot well estimate the continu- 

 ance of the present level along that shore at less than ten thous- 

 and years ; it probably exceeds twice that period, as is shown by 

 the amount of cutting done by the sea along the rocks which are 

 now exposed to wave action. For this great period of time the 

 delicate kames which extend down to within twenty feet of high- 



