1879.] 281 [Shaler. 
of very strong currents,! for we everywhere have a more or less dis- 
tinct cross-bedded structure. To suppose that the erosion of these ma- 
terials occurred at the time of their deposition, or that they were de- 
posited simultaneously throughout the area, seems to me quite unrea- 
sonable. The only way that I can solve the problem of its formation 
is by assuming that this mass of detritus originally occupied a coast- 
shelf. In the final elevations which converted a large part of this re- 
gion into the land on which the coal forests grow, this mass of waste 
was torn up by the waves, and swept rapidly out to sea by tidal and 
other currents. 
This hypothesis has the considerable advantage that it perfectly 
accounts for the rapid invasion of the waste into regions hitherto 
deep sea. ‘The lifeless nature of the deposit would be a consequence 
of its relatively rapid movement, for it is a well-known fact that 
rapidly moving sands and gravels do not readily become occupied by 
marine life. The hypothesis of elevation accounts, moreover, for the 
great amount of cross-bedding shown in these rocks. None of these 
facts can be met by any other hypothesis that has been suggested. 
It is readily seen that this method of interpreting the record which 
comes to us in our detrital formations, brings us in face of certain 
difficulties. Where we have a deposit laid down i1 the deep sea, we 
can safely assume that beds at corresponding heights above its base 
are synchronous, but in a deposit made in the fashion of our coast 
shelves, the succession of deposition is not the same. The part near- 
est the shore is deposited before the seaward part. The deposit is 
then essentially like a cross-bedded stratum on a large scale. Owing, 
however, to the great horizontal extent of the bed, it will not often 
be possible to prove the cross-bedding as we can when it is exhibited 
in a limited way. For instance, in the Kentucky district before 
described, the millstone grit has always been considered as of the 
same age in all parts of the area, but if I am correct, a long time 
must have elapsed after the deposit began in the eastern part of the 
strata, before the beds of the same stratum were laid down in the 
western part of the area, a time during which the land was lifted at 
least one thousand feet in altitude, and the coast-shelf pushed out 
over four hundred miles beyond its former position. 
1 The east and west sections, across this area of millstone grit, has a length of 
about four hundred miles, though erosion has divided it into several areas. The 
north and south extension is considerably greater. 
