38 
POPULAR SCIENCE REVIEW. 
manner immense blocks of stone detached by frost from the 
rocks within the Arctic circle are annually floated from the 
Polar seas southward in the ice which is attached to them^ and 
then dropped when the ice mass melts in a warmer climate. 
These icebergs are sometimes found floating in masses of 
incredible size^ even half or two-thirds of a mile in lengthy and 
sunk in the sea to the depth of 1_,000^ 1^500_, and 2_,000 feet. 
The base of glaciers is usually thickly set with fragments of 
rock_, pebble^ and coarse sand^ firmly frozen into the icy mass^ 
which acts as a huge rasp to the underlying rocks^ scratching 
or striating their surfaces in moving over them^ or as a smooth- 
ing polishing instrument^ if the earthy materials in the ice are 
finely comminuted. This fact is of great importance in a geo- 
logical point of vieWj as it has led to the identification of the 
phenomena of erratic blocks_, of smoothed_, striated_, crushed 
surfaces_, which appear both contiguous to and often /cw’ remote 
from existing glaciers. These phenomena are frequently ob- 
served in the United Kingdom^ and well known to English 
geologists. The lowlands of Britain and Ireland are thus 
proved to be portions of the bed of an ancient glacial sea ; and 
geological inquiries have further established the fact_, that when 
the bed of this sea was elevated^ the dry land extended over 
to the north-west of Grermany_, from which our islands again 
became divided_, by a subsequent disruption resulting in the 
formation of the Irish Sea_, the St. George^s Channel_, and the 
German Ocean. 
The above facts respecting the English flora^ and the attempt 
to account for them by reference to past changes^ open a wide 
field for philosophic inquiry and research. Every country 
presents similar phenomena in the distribution of its plants 
which cannot be explained by any other hypothesis. 
A great many plants that are found in the British Isles_, for 
example^ are indigenous in the State of Pennsylvania^ Uortk 
America. My reasons for believing this are simply these^ I 
have found them myself under circumstances that would indi- 
cate that they are natives. The woods of Pennsylvania are 
the natural growths of the soil, and in these woods, miles from 
human habitation, I have found British plants belonging to 
species without organised contrivance in the seed for effecting 
their diffusion to localities at a distance from the parent plant. 
When I find, for example, in the cranberry bogs, in the neigh- 
bourhood of Philadelphia, Pennsylvania, both our sundews, 
Drosera {rotundifoUa and longifoUa) , and our species of bog 
moss {Sphagnum acutifolium), in the salt marshes of Staten 
Island in the New York Bay, such English plants as glasswort 
{8a Ucornia herhacea), sea knot-grass {Polygonum maritimum) 
sea-plantain {Plantago maritima), and our English sand-reed 
