200 _ GEOGNOSY AND GEOLOGY. 
and Blackgang cave (fig. 5) on the Isle of Wight; and the peculiar arch 
of rock on Cape Parry, in the arctic regions ( fig. 3). Similar formations 
occur in the case of fresh water streams and lakes. ! : 
Waterfalls, too, in particular cases, produce striking results. Thus the 
entire body of water in Lake Erie, in pouring into Lake Ontario, dashes 
over a precipice of about 160 feet in height. The rock wall over which the 
water pours is continually undermined by the impact of 670,000 tons of 
water in every minute falling from the top, and the upper portion crumbles 
oradually away, so that the falling mass constantly recedes in position. At 
some future day the recession may extend to Lake Erie, and the result may 
be the draining of the lake itself, or even of the entire lake series, thus 
adding nearly 72,930 square miles to the land surface; a cataclysm of no 
ordinary magnitude when, in addition to the above result, we consider the 
effect which must be produced by the impetuous bursting of all their barriers 
by the waters in the descent to the sea. PJ. 50, fig. 8, represents the 
Niagara Falls from the American side. Similar phenomena are presented 
by the Dal-Elf-Fall near Elfkarleby in Sweden (fig. 9), as also by the 
Rhonetrichter near Bellegarde in the French department de |’ Ain ( pl. 53, 
jig. 6). Streams of water sometimes often pierce rocks and form great 
gateways, over which pass the so-called natural bridges, constituted by the 
portion remaining. A remarkable instance of this is to be found in the 
valley of Icomonzo in Columbia ( pl. 49, fig. 4). The natural bridge near 
Lexington, Virginia, is another illustration. 
Water in the form of ice often produces great disturbing effects, and, 
indeed, has every title to being considered as a rock species. The descent 
of large glacier masses to the edge of the sea, and their accumulation along 
the shore, give rise to Icebergs, which are sometimes very dangerous 
to navigation, both in their original locality and in more tropical regions, to 
which they are carried by ocean currents or winds. It is to glaciers and 
icebergs that many of the phenomena of diluvial scratches, transportation 
of boulders and rocks, &c., are ascribed. Pl. 52, fig. 1, is a view of 
icebergs and ice-cliffs in the antarctic regions. 
ABNORMAL ROCK MASSES. 
Abnormal masses are essentially different from the normal, in standing in 
regular order of succession neither to the latter nor to each other. They 
pierce through the normal rocks in the most diversified directions, and 
traverse them just as they traverse each other. In this interpenetration of 
each other by abnormal masses, it is possible to decide in many cases as to 
the relative ages; but the determination is always more difficult than in the 
case of normal rocks, and the same species may in one region be older, and 
in another younger, than those with which it is associated. This relation is 
beautifully exhibited by granite, which was considered by the older 
geologists to be the most ancient of all rocks. This supposition is most 
certainly true in many cases, yet granite is known more recent than the 
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