FRESH-WATER DEPOSITS. 
363 
sediment; for, if long exposed after death, the membrane con¬ 
taining the ink would have decayed.* 
As we know that river-fish are sometimes stifled, even in 
their own element, by muddy water during floods, it can not 
be doubted that the periodical discharge of large bodies of 
turbid fresh water in the sea may be still more fatal to ma¬ 
rine tribes. In the ‘^Principles of Geology” I have shown 
that large quantities of mud and drowned animals have been 
swept down into the sea by rivers during earthquakes, as in 
Java in 1699 ; and that indescribable multitudes of dead fish¬ 
es have been seen floating on the sea after a discharge of 
noxious vapors during similar convulsions. But in the inter¬ 
vals between such catastrophes, strata may have accumu¬ 
lated slowlv in the sea of the Lias, some being: formed chief- 
ly of one description of shell, such as ammonites, others of 
gryphites. 
Fresh-water Deposits.—Insect-beds. — From the above re¬ 
marks the reader will infer that the Lias is for the most part 
a marine deposit. Some members, however, of the series 
have an estuarine character, and must have been formed 
within the influence of rivers. At the base of the Upper and 
Lower Lias respectively, insect-beds appear to be almost 
everywhere present throughout the Midland and South-west¬ 
ern districts of England. These beds are crowded with the 
remains of insects, small fish, and crustaceans, with occasion¬ 
al marine shells. One band in Gloucestershire, rarely ex¬ 
ceeding a foot in thickness, has been named the “ insect lime¬ 
stone.” It passes upward, says the Rev. P. B. Brodie,f into 
a shale containing Cypris and Estheria^ and is full of the 
wing-cases of several genera of coleoptera, with some nearly 
entire beetles, of which the eyes 
are preserved. The nervures of 
the wings of neuropterous insects 
(Fig. 382) are beautifully perfect 
in this bed. Ferns, with cycads 
and leaves of monocotyledonous 
plants, and some apparently brack- „ Nat. size, 
r, 1 /- 1 Wing ora neuropterous insect, from 
ish and iresh-water shells, accom- the Lower Lias, Gloucestershire. 
pany the insects in several places, (Rev. p. b. Brodie.) 
while in others marine shells predominate, the fossils varying 
apparently as we examine the bed nearer or farther from the 
ancient land, or the source whence the fresh water was de¬ 
rived. After studying 300 specimens of these insects from, 
the Lias, Mr. Westwood declares that they comprise both 
* Buckland, Bridgewater Treatise, p. 307. 
t A History of Fossil Insects, etc., 1846. London. 
Fig. 382. 
