a. 
322 PROCEEDINGS OF THE GEOLOGICAL sociETy. [Apr. 19, 
the power by some means of sorting the pebbles which lie at the 
bottom, their size decreasing with surprismg regularity, even till 
they pass into sand, with the increasing depth. There is some diffi- 
culty in understanding how this is effected: Playfair has suggested 
that the undulations of the sea propagated downwards from the sur- 
face, tend to lift up and down the pebbles at the bottom, and that 
such are liable, when thus quite or partially raised, to be moved on- 
wards even by a very weak current. Should, therefore, a boulder 
formation be exposed during subsequent changes of level to the 
action of the sea, pebbles derived from it, and decreasmg m size with 
perfect regularity according to their distance from their source, 
might be thus spread out. Hence I conceive that from a group of 
mountains, which had once existed as an island, boulders, decreasing 
in bulk with some degree of regularity, and beyond them pebbles 
degenerating with perfect regularity into sand, might be spread out, 
thus simulating the effects of a great debacle, which m rushing 
along had insensibly lost its power, and yet that both boulders and 
pebbles had been transported by the ordmary currents of the sea; 
aided, in the one case, by floating coast-ice, in the other, apparently 
by the undulatory movement of the water. 
The two objections, therefore, which have been here discussed, 
cannot, I think, any longer be considered as absolutely fatal to the 
theory of floating ice; and thus far the hypothesis of a debacle is 
no longer necessary. 
If the explanation here given of the transportal of boulders from a 
lower to a higher level be hereafter proved correct, we gain, im all 
cases where the horizontal distance between the boulders and the 
parent rock is not so great as to allow of the probability of subse- 
quent unequal movements of elevation, a valuable measure of subsi- 
dence during a defined period. We are accustomed to precise 
measurements of elevation, from the ascertained heights of upraised 
marine remains; but it seemed quite hopeless to expect this, even in 
a lesser degree, with respect to subsidence,—that movement which 
hides under the sea the surface affected. It is marvellous that 
Nature should have thus marked by buoys made of stone, the former 
sinking of the earth’s crust, and likewise, I may add, its subsequent 
elevation ; and that on these blocks of stone the temperature, during 
the long period of their transportal, may be said to be plainly engraved. 
Moreover, it is thus shown that the subsidence durmg no one entire 
summer was so great as to carry the coast-boulders beneath that 
small limit of depth at which the salt water durmg each ensuing 
winter became frozen. 
Note.—Atfter this paper was read, Mr. ‘Nien objected that when 
the parent rock was once submerged, no further supply of boulders 
could be derived from it, and consequently if afterwards, each time 
they were afloat, only one boulder out of a hundred was dropped in 
water too deep for it to be refrozen in the coast-ice, after a certain 
time there would be none left to be carried up, during the continued 
subsidence, to the higher levels. This appears to me an objection of 
much force. I would, however, remark in the first place, that I do 
