344 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROTTE 



was written, but recently communicated to the writer by Dr Gill, that 

 the sharks of lake Nicaragua are specifically identical with those found 

 in adjacent portions of the Pacific ocean, but distinct from those found 

 in the Caribbean sea. 



SUBSEQUENT MODIFICATION OF THE LAKE 



Volcanic eruptions. — The original outline of the lake formed behind the 

 barrier of volcanic ejecta was probably quite different from that of the 

 present lakes. The subsequent modification has been due to several 

 agencies. The continuation of volcanic eruptions has doubtless very 

 much contracted the northwest portion of the depression. It is prob- 

 able that the original depression was occupied by a single lake which 

 extended northwestward beyond the present limits of lake Managua. 

 Later eruptions encroached on this portion of the lake basin, and finally 

 a flood of volcanic ash and mud was carried entirely across the depres- 

 sion, forming a barrier which cut off the upper portion of the lake, raising 

 its surface between 30 and 40 feet above the surface of the larger portion 

 to the southeast. The strip of land separating the two lakes is a nearly 

 perfect plain composed of partially consolidated volcanic tuff. 



The Tipitapa river, which forms the outlet of lake Managua, crossing 

 this barrier, has cut its channel backward nearly to the upper lake. It 

 falls about 13 feet within less than half a mile of the point where it 

 emerges from lake Managua. In a very short time, therefore, unless 

 the backward cutting of this stream is arrested, the level of lake Managua 

 will be lowered to the extent of 13 feet. 



The original outline of lake Nicaragua has been further slightly modi- 

 fied by the recent volcanic eruptions in the vicinity of jNIadera and 

 Ometepe, and perhaps also of Mombacho. The northeast side of the 

 latter volcano appears to have suffered an enormous landslide, which 

 has pushed before it a great mass of earth and rock. This now has a 

 peculiar hummocky surface and forms a long point projecting into the 

 lake and a large number of small islands. 



Wave cutting. — The outline of the lake has further been modified by 

 the action of the waves. The trade winds produce a nearly constant surf 

 on its west side, and this has accomplished considerable erosion at cer- 

 tain points. The wave action has probably cut a shelf into the adjoin- 

 ing plain entirely around this portion of the lake, the extent of the shelf 

 depending on the character of the rocks which were encountered b}' the 

 waves. 



In the region south of Madera bold headlands are formed by masses of 

 hard igneous rocks which tend to protect the less resistant rocks between. 



