48 LYELL'S ELEMENTS OF GEOLOGY. 



Distinction of Freshwater from Marine Formations. 



Lamarck (see Fig. 19. p. 41.,) inhabits salt water; and, although 

 the animal differs shghtly, the shell is undistinguishable from that 

 of the Cypris. 



The seed-vessels of Chara, a genus of aquatic plants, are 

 very frequent in freshwater strata. These seed-vessels were 

 called, before their true nature was known, gyrogonites, and 

 were supposed to be shells. (See Fig. 49. a.) 



The Charge inhabit the bottom of lakes and ponds, and 

 flourish mostly where the water is charged with carbonate of 

 lime. Their seed-vessels are covered with a very tough integu- 

 ment, capable of resisting decomposition ; to which cu'cumstance 

 we may attribute their abundance in a fossil state. The annexed 

 figure (Fig. 50.) represents a branch of one of many new spe- 

 cies found by Professor Amici in the lakes of northern Italy. 

 The seed-vessel in this plant is more globular than in the British 

 Charse, and therefore more nearly resembles in form the extinct 

 fossil species found in England, France, and other countries. 

 The stems, as well as the seed-vessels, of these plants are found 

 both in modern shell marl and in ancient freshwater formations. 

 They are generally 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. 49.) 



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

 impressions of leaves, and branches of trees, in strata containing 

 freshwater shells ; and we also find occasionally the teeth and 

 bones of land quadrupeds, of species now unknown. The man- 

 ner by 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 the fish are occasionally useful in determining 

 the freshwater origin of strata. Certain genera, such as carp, 

 perch, pike, and loach, {Cyprinus, Perca, Esox^ and Cobitis,) 

 as also Lebias, being peculiar to freshwater. Other genera con- 

 tain 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 

 existing fishes, that it is very difficult, at least in the present state 

 of science, to derive any information from icthyolites, respecting 

 the element in which strata were deposited. 



* See Index, " Fossilization." 



