32 



FEESHWATER AND MAEINE FORMATIOXS. 



[Ch. iir. 



stems, as well as tlie seed-vessels, of tliese plants occur both in modern 

 shell marl and in ancient freshwater formations. They are generally 



Tiff. 53. 



riff. 54. 



Chara medicagimda ; 

 fossil. Upper Eocene, 

 Isle of Wight. 

 a. Seed-vessel, 



magnified 20 



diameters. 

 5. Stem, magnified. 



C7iara elasUea ; recent Italy. 



Sessile seed-vessel between the divisions of 



the leaves of the female plant. 

 Magnified transverse section of a branch, 



with five seed-vessels, seen from below 



upwards. 



composed of a large tube surrounded by smaller tubes ; the whole stem 

 being divided at certain intervals by transverse partitions or joints. 

 (See b, fig. 53.) 



It is not uncommon to meet with layers of vegetable matter, impres- 

 sions of leaves, and branches of trees, in strata containing freshwater 

 shells ; and we also find occasionally the teeth and bones of land quad- 

 rupeds, of species now unknown. The manner in which such remains 

 are occasionally carried by rivers into lakes, especially during Hoods, has 

 been fully treated of in the " Principles of Geology.'"^ 



The remains of fish are occasionally useful in determining the fresh- 

 water origin of strata. Certain genera, such as carp, perch, pike, and 

 loach ( Cyprinus, Perca^ £Jsox, and Cobitis), as also Zebias, being pe- 

 culiar to freshwater. Other genera contain some freshwater and some 

 marine species, as Coitus^ Mugil, and Anguilla^ or eel. The rest are 

 either common to rivers and the sea, as the salmon ; or are exclusively 

 characteristic of salt water. The above observations respecting fossil 

 fishes are applicable only to the more modern or tertiary deposits ; for 

 in the more ancient rocks the forms depart so widely from those of ex- 

 isting fishes, that it is very difiicult, at least in the present state of sci- 

 ence, to derive any positive information from icthyolites respecting the 

 element in which strata were deposited. 



The alternation of marine and freshwater formations, both on a small 

 and large scale, are facts well ascertained in geology. When it occurs 

 on a small scale, it may have arisen from the alternate occupation of 

 certain spaces by river water 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 returns, 

 and, on resuming its former place, brings with it sand, mud, and marine 

 shells. 



* See Index of Principles, " Fossilization." 



