318 PROCEEDINGS OF THE GEOLOGICAL SocieTy. _[ Apr. 19, 
the ice under water had decreased in thickness in the interval of the 
boulders having been caught up and dropped. In any case the notion 
of icebergs having, in the States of New York, New England, and 
northern Pennsylvania, lifted up numerous boulders from a depth of 
between 1000 and 2000 feet, is quite madmissible. 
In my paper on the Boulders of the Southern hemisphere, read be- 
fore this Society *, I pomted out that there were two methods, essen- 
tially distinct both in the requisite climate and in the results produced, 
by which fragments of rock are transported ; namely, by icebergs and 
by coast-ice. Icebergs now transport fragments of rock on the west 
coast of South America, in the latitude of the central parts of Eu- 
rope, under a temperate climate where the sea, even in protected bays, 
is never frozen. On the other hand, in the northern parts of the 
United States and in the Gulf of Bothnia, where the climate is ex- 
cessive, but yet under a latitude where glaciers never descend to the 
level of the sea, fragments of rock are annually enclosed by the 
freezing of the coast-water, and are thus transported. In the polar 
regions both actions concur. Icebergs will transport such fragments 
of rock as fall on the parent glaciers, and these are generally quite 
angular. From the vast size of the bergs, the fragments will often 
be transported to great distances, and when deposited, it must be in 
deep water, and therefore (as well as from the original descending ~ 
movement of the glacier) at a much lower level than the parent rock : 
when once dropped, they will probably never again be moved by ice. 
On the other hand, coast-ice will transport whatever fragments of 
rock or pebbles lie on or near the shore. These fragments, from 
being repeatedly caught im the ice and stranded with violence, and 
from being every summer exposed to common littoral action, will 
generally be much worn; and from being driven over rocky shoals, 
probably often scored. From the ice not being thick, they will, if 
not drifted out to sea, be landed in shallow places, and from the 
packing of the ice be sometimes driven high up the beach, or even 
left perched on ledges of rock. By this agency boulders will proba- 
bly not be carried to such great distances as by icebergs, and the 
limit of their transportal will perhaps be more defined. In South 
America there is a considerable difference in the state of the 
boulders in Tierra del Fuego, where a large proportion are much 
rounded, and on the plains of Patagonia and in Chiloe further from 
the pole, where the boulders are larger and quite angular. I attri- — 
buted the presence of these latter to the exclusive action of icebergs ; 
whilst in Tierra del Fuego coast-ice appears formerly to have come 
into play. On Moel Tryfan+ the well-rounded fragments of chalk- 
flints were in all probability transported by coast-ice: though I ean- 
not doubt, from the extraordinary manner in which the lamine of 
the slate rocks have there been shattered, that icebergs have likewise 
been driven against them when under water; so that both actions seem 
there to have concurred. Some other distinguishing characters be- 
tween the action of coast-ice and of icebergs will presently be poimted 
* Transactions of the Geol. Soc. vol. vi. (second series) p. 430. 
+ London Philosophical Journal, vol. xxi. p. 186. 
