302 C. W. HAYES — GEOLOGY OF NICARAGUA CANAL ROUTE 



canic vents forming more or less isolated mountains. Of these Ometepe, 

 Masaya, Momotombo, and several otiiers to the northwest of tlie latter 

 have been in eruption within historic times. Others are in the solfataric 

 stage, while still others appear to be entirely extinct. The group of peaks 

 between Momotombo and Coseguina is called the Maribios range. 



It is probable that the vents which formed the Costa Rican range broke 

 out upon a soniewhat elevated plateau, while those which formed the 

 Nicaraguan range broke out on the sea bottom. The latter, also, are far- 

 ther apart, except those northwestward of Momotombo, which form the 

 Maribios range. This may explain the greater hight and massiveness 

 of the Costa Rican range, and the amount of material erupted from the 

 two series of vents may not differ greatly. 



As already indicated, it is probable that the form of the coast has been 

 materially modified by this recent volcanic activity. The whole of the 

 countrj' between the northern portion of lakes Nicaragua and Managua 

 and the Pacific consists entirely of recently ejected volcanic material, 

 and the region which it now occupies was doubtless a portion of the 

 Pacific until recent geologic times. The former coastline is rejjresented 

 on the sketch map forming plate 30. 



The surface of this newly added land is composed of level or gentl}'- 

 sloping plains, isolated conical volcanic peaks, and the more crowded 

 peaks of the Maribios range. T3^pes of the entirely isolated peaks are 

 Ometepe and Momotombo. Both of these are composed of alternate 

 layers of lava and ash. The latter, however, gives them their perfect 

 conical form. Both have been in eruption within historic times, and 

 considerable smoke still comes from Momotombo. Only a small amount 

 of steam and sulphurous vapors are at present emitted from the crater 

 of Ometepe. 



Modification by the ordinary processes of erosion, in the form of these 

 steep cones of unconsolidated ash, is extremel}'' rapid, and their summits 

 vary in detail of outline from year to year. Madera and Zapatera are 

 volcanoes, which have been extinct for some time, and the agents of 

 degradation have materially reduced their hight and destroyed the orig- 

 inal conical form of their summits. The unconsolidated ash has been 

 largely removed from their upper portions, leaving onl}^ the massive lava 

 beds in place ; hence their formation has been ascribed to a different 

 form of eruption from that which produced Ometepe and Momotombo. 

 It is probable, however, that the summits of the former once consisted 

 of ash cones, and that the eruptions in all have been accompanied by 

 more or less explosive violence, to which the unconsolidated fragmental 

 material is due. The lower slopes of Mombacho are rather smooth and 

 symmetrical ; but instead of a single cone its summit is truncated and 



