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much greater portion is raised from the bottom than reaches the
surface of the lake. Several large rents or cracks may be seen in
the earth about the place, which appear to have been occasioned
by its stretching to reach the surface. It never rises far above
the level of the lake; but having once attained the surface, it
for a time, fluctuates with the rise and fall of the water; after
which it sinks gradually. When at rest in the bottom of the lake
it has the same appearance as the neighbouring parts, being
covered with the same vegetation, consisting principally of
Littorella lacustris, interspersed with Lobelia
dortmanna, Isoetes lacustris, and other plants common
in this and all the neighbouring lakes: after remaining some time
above the water its verdure is much improved. For a few inches in
depth it is composed of a clayey or earthy matter, apparently
deposited by the water, in which the growing plants have fixed
their roots; the rest is a congeries of decayed vegetable matter
forming a stratum of loose peat earth about six feet in
thickness; which rises from a bed of very fine soft clay. A
considerable quantity of air is contained in the body of the
island, and may be dislodged by probing the earth with a pole.
This air has been found by Dr. Dalton to consist of equal parts
of carburetted hydrogen and azotic gasses, with a little carbonic
acid.
For the last quarter of a century the times of its appearance
have been as follows. In 1808 from the 20th July to the beginning
of October; in 1813
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