240 PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 



until the divide was pushed back to its present position at Es- 

 pinal. Streams to the northward also suffered some ] s of ter- 

 ritory. Thus upper tributaries of the Medio and Gourdes were 

 diverted, forming portions of the present Guachipilin, Matinga, 

 and Chacalapa. The deserted river valley formerly occupied by 

 the eastward-flowing Tola and Cascabel, and from which the be- 

 headed Guiscoyol now flows, forms the lowest gap in the conti- 

 nental divide. Its summit is 154 feet above sea-level, and the 

 ascent is so gradual from either side that accurate instrumental 

 work is required to locate the divide. During the wet season the 

 gap is occupied by a swamp from which the water appears to 

 flow both to the Atlantic and Pacific. 



The map (plate 6), on which the present and former coast 

 lines are represented, shows that a large land area has recently 

 been added to this portion of the isthmus. The topography of 

 this region differs totally from that of the Nicaraguan depres- 

 sion above described. The latter is an oldland, and its surface 

 forms are those developed by the processes of subaerial degra- 

 dation. The former, on the other hand, is composed of mate- 

 rials recently ejected from volcanic vents, and it retains to a large 

 extent its original constructional forms. 



Extending through the center of the area in question is a line 

 of volcanoes, all of which are geologically very recent, while 

 some are still active. From these numerous vents, which were 

 at first submarine, a vast amount of material has been erupted, 

 building up broad, gently sloping plateaus, from which rise more 

 or less isolated volcanic cones. The southernmost of these vol- 

 canoes are the twin peaks of Madera and Ometepe, which occupy 

 an island in Lake Nicaragua. Toward the northwest are Za- 

 patero, also on an island, Mombacho, Masaya, Chiltepe, Momo- 

 tombo, the clustered peaks forming the Maribios range, and 

 finall}' Coseguina, on the Gulf of Fonseca, Most of these vol- 

 canoes have erupted both molten lavas and fragmental material, 

 the latter varying from coarse blocks of solid lava to the finest 

 dust. It is this fragmental material which gives to these vol- 

 canoes their beautifully symmetrical forms. Those which have 

 been recently active, as Ometepe and Momotombo, are almost 

 perfect cones. The effects of erosion, however, are seen almost 

 before the volcanic activity ceases, and the symmetry of the 

 cones is quickly destroyed. The details of outline in the ash 

 cones vary from year to year. When the loose material has 

 been removed or modified by erosion the summits assume the 



