FRESH-WATER AND MARINE FORMATIONS. 
59 
for in the more ancient rocks the forms depart so widely 
from those of existing fishes, that it is very difficult, at least 
in the present state of science, to derive any positive infor¬ 
mation from ichthyolites respecting the element in which 
strata were deposited. 
The alternation of marine and fresh-water formations, both 
on a small and large scale, are facts well ascertained in ge¬ 
ology. When it occurs on a small scale, it may have arisen 
from the alternate occupation of certain spaces by river-wa¬ 
ter and the sea; for in the flood season the river forces back 
the ocean and freshens it over a large area, depositing at the 
same time its sediment; after which the salt water again re¬ 
turns, and, on resuming its former place, brings with it sand, 
mud, and marine shells. 
There are also lagoons at the mouth of many rivers, as the 
Nile and Mississippi, which are divided off by bars of sand 
from the sea, and which are filled with salt and fresh water 
by turns. They often communicate exclusively with the 
river for months, years, or even centuries; and then a breach 
being made in the bar of sand, they are for long periods filled 
with salt-water. 
Lym-Fiord. —The Lym-Fiord in Jutland offers an excellent 
illustration of analogous changes; for, in the course of the 
last thousand years, the western extremity of this long frith, 
which is 120 miles in length, including its windings, has been 
four times fresh and four times salt, a bar of sand between it ^ 
and the ocean having been often formed and removed. The 
last irruption of salt water happened in 1824, when the North 
Sea entered,killing all the fresh-water shells,fish, and plants; 
and from that time to the present, the sea-weed Fitcus vesi- 
culosiis^ together with oysters and other marine mollusca, 
have succeeded the Cyclas^ Lymnma^ Paliidina^ and Charmj^ 
But changes like these in the Lym-Fiord, and those before 
mentioned as occurring at the mouths of great rivers, will 
only account for some cases of marine deposits of partial ex¬ 
tent resting on fresh-water strata. When we find, as in the 
south-east of England (Chap. XVIII.), a great series of fresh¬ 
water beds, 1000 feet in thickness, resting upon marine for¬ 
mations and again covered by other rocks, such as the creta¬ 
ceous, more than 1000 feet thick, and of deep-sea origin, we 
shall find it necessary to seek for a different explanation of 
the phenomena. 
* See Principles, Index, “Lym-Fiord.” 
