32 



FEESHWATER AND MARINE FORMATIONS. 



[Ch. III. 



stems, as well as the seed-vessels, of these plants occur both in modem 

 shell marl and in ancient freshwater formations. They are generally 



Fig. 53. 



Fig. 64. 



Chara meclicaginula ; 

 fossil. Upper Eocene, 

 Isle of Wight. 

 a. Seed-vessel, 



magnified 20 



diameters. 

 6. Stem, magnified. 



Chara elastica ; recent. Italy. 



a. Sessile seed-vessel between the divisions of 

 the leaves of the female plant. 



5. 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 floods, 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, Esox, and Cobitis), as also Lebias, being pe- 

 culiar to freshwater. Other genera contain some freshwater and some 

 marine species, as Cottus, 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 difficult, 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 

 6hells. 



* See Index of Principles, " Fossilization." 



