ORIGIN OP LAKE NICARAGUA 343 



the latter ; that the backward cutting of the east-flowing stream lowered 

 a gap in the divide, and by reason of the less resistant material of which 

 it was composed diverted at first a part and finally all the waters of the 

 lake to the east. This is only a modification of the third hypothesis, 

 and does not affect the main point, namely, that for a longer or shorter 

 period the lake had two outlets — one by the Lajas-Grande gap and the 

 other by the valley of the present San Juan. 



This modification of the hypothesis removes one of the most serious 

 objections to the above stated theory for the origin of the lakes. An ex- 

 amination of the region which it assumes to have been occupied by the 

 Continental divide leads to the conclusion that the lowest gap in the 

 divide was probably more than 50 feet above the present river. An ele- 

 vation for the present divide above the San Juan at Castillo of 100 feet 

 or possibly more would accord better than an elevation of 50 feet or less 

 Avith the topography and drainage of the region and with the character- 

 istics of divides in general; and it is by no means impossible that the 

 backward cutting of the eastward flowing stream should lower the gap 

 50 or 75 feet in residual clay while the outlet of the lake was cutting the 

 4 or 5 miles of rock gorge new occupied by the Rio Grande. 



The above theory for the formation of lake Nicaragua is supported by 

 a consideration of its fauna. In a paper on the fishes of the lake. Gill 

 and Bransford make the following statements : ^ 



" The element of especial interest in connection with the ichthyic fauna of the 

 lake is the association of forms that we are in the habit of regarding as character- 

 istically marine with those that are at least as exclusively fresh- water types. Thus 

 with the species of Cichlids and Characinids, of which no representatives have 

 been found in marine waters, we have a species of Megalops, a shark and a sawfish." 



"The why and wherefore of such combinations of species are not entirely ap- 

 parent. They may have resulted (1) from the intrusion of salt-water types into 

 the fresh waters, or (-2) from the detention and survival of the salt-water fishes in 

 inlets of the sea that have become isolated and gradually become fresh-water lakes. 

 On the whole, it appears more probable that the latter is the case. By the uplift 

 of the land an inlet of the Pacific ocean might have been shut off from communi- 

 cation from the ocean, and the character of the water would be soon changed by 

 the copious showers of that tropical country. The shark, sawfish, megalops, and 

 other species mostly found in the sea had, however, time to accommodate them- 

 selves to the altered conditions, and in this connection it must be remembered, 

 too, that most of the types in question are known to voluntarily ascend high up 

 streams and even into fresh water. The numerous rapids of the river discharging 

 from the lake discourage, however, the idea that the species enumerated have vol- 

 untarily ascended that river and entered the lake." 



Of special significance is the fact, not known at the time the above 



* Theodore Gill and J. F. Bransford: Synopsis of the Fishes of Lake Nicaragua. Proc. Acad. 

 JJat. Sci., Philadelphia, vol. xxix, 1877, p. 179. 



