758 ON THE GEOLOGY OF THE ISLAND OF AQUIDNECK. 
of stratified drift would have become covered with a luxuriant 
vegetation which was in time to be converted into the beds of coal 
in which its remains have been entombed. The present geological 
period is a carboniferous period, and in time its coal beds will be 
found resting upon just such a section as now characterizes the 
rocks of the ancient coal time. The Dismal Swamp, when it is 
converted into coal, as it well may be in the future, will show 
a drift section beneath it where the conglomerates will be com- 
posed of pebbles which owe much of their transportation to ice 
action, though their final arrangement is the work of water. It 
may be asked why do we not have the unstratified drift and the 
scratched pebbles of the glacial period which preceded the carbon- 
iferous epoch; the answer is easy to find, the shore regions of any 
continent when the successive submergences keep up the process 
of deposition, are the only parts of its surface where we can expect 
to find a record of ancient conditions long before anything like the 
time has elapsed which has rolled away since the carboniferous 
period, or the unstratified drift of our shores may have disappeared, 
leaving only such imperfect record as may be perceived in the bed- 
ded conglomerate which may happen to be buried beneath succeed- 
ing deposits. The conjunction of conglomerates and coal beds is 
not limited to the carboniferous period. Iam unacquainted with 
the history of the jurassic and cretaceous coals which occur at 
various points, but in the tertiary period we see at least twice the 
same swift change from the desolation of glacial conditions to lux- . 
uriant vegetation, which is shown in the period in which we now 
live, and which I have suspected in the carboniferous time. 
` The cause of these sudden transitions in climatic conditions 
is yet to be explained. Apart from the question of the origin o 
the glacial periods of the past which cannot be discussed here, it is 
easy to see that the glacial period which has just passed away ine 
done much to favor the development of a luxuriant vegetation 
over a large part of the country it affected. In the first place the 
ice work of the glacial time was effective in producing a large 
amount of well ground material. The surfaces it covered were 
probably reduced to a state fit for assimilation by plants at a very 
much more rapid rate than would else have taken place in the 
same regions under the existing conditions ; it must also be noted 
that the supply of nutriment from the rocks is very much more rapid 
on a soil filled with glacial material than in one where the action 
