in Derwent Lake, Keswick. 65 
are irregular and uncertain; it has sometimes 
been observed in two successive years; at 
other times with aninterval of seven or eight 
years ; and mostly, I believe, about the termi- 
nation of a warm, dry season. Its figure 
and extent are also variable; it has some- 
times contained’ near an acre of ground, at 
other times only a few perches: and when at 
rest in the bottom of the Lake, it cannot be 
distinguished’ from the adjoining parts: in 
fact, it is not always the very same piece of 
earth’ that forms the Island. 
It is‘entirely covered with vegetation, prin- 
cipally the Isoetes lacustris, interspersed with 
Lobelia dortmanna, and other plants com- 
monly found growing in the bottom of this 
and ‘all the neighbouring lakes; its surface, 
to' the depth of a few inches, is composed of 
a clayey or earthy matter, apparently depo- 
sited by the water; the rest is'a kind of im- 
perfectly formed peatmoss ; the species of some 
of the vegetables composing it, may be still re- 
cognised, and appear of kinds not now grow- 
ing upon the spot, but probably accumulated 
at a remote period when the surface of the 
lake was much lower than at present: a con- 
jecture which is countenanced by the appear- 
ance of the roots of large oak and other trees 
yet remaining in different parts of the lake, 
VOL. III. a 
