304 C. W. HAYES — GEOLOGY OF NICAKAGUA CANAL ROUTE 



point of the surroundinf;; rim and about 1,500 feet below the highest point 

 of the rim. This highest point probably coincides very nearly with the 

 former volcanic peak, although the latter, being composed ahnost entirely 

 of unconsolidated ash, has been very much reduced in hight by erosion. 

 Several other caldera lakes of this type occur in the vicinity of Managua. 

 Volcanic plateaus. — Reference has been made to a plateau lying south- 

 west of lake Managua and the northern end of lake Nicaragua, which I 

 have called the Jinotepe plateau, from the i)rincipal town on it. This 

 plateau is composed entirely of recently ejected volcanic material, chiefly 

 a partially consolidated volcanic tuff, .which was spread out probably in 

 the form of a semi-liquid mud. The plateau has an altitude along its 

 northeastern margin of 1,200 to 1,800 feet. From this gently undulating 

 summit it descends gradually south and southwest to its margin against 

 the older rocks to the south and to the Pacific coast. The central por- 

 tion of the plateau has been but little modified by erosion, and probably 

 preserves very nearly its original constructional form. This is due largely 

 to the porous nature of the volcanic ash of which the surface is composed. 

 The rain waters sink into the ground before they have an opportunity to 

 collect into sufficient volume to effect any modification of the surface, 

 except where the original slopes were very steep. A belt along the coast, 

 however, has 'been rather deeply dissected by stream channels, where 

 the smaller intermittent tributaries are collected into permanent trunk 

 streams and where the plateau has a decided seaward slope. Toward the 

 north and northeast the plateau is terminated by a somewhat abrupt 

 e3carpment, which separates it from the lower plain of Leon and from 

 the plain lying between lakes Managua and Nicaragua. These lower 

 plains have precisely the same origin as the Jinotepe plateau, and it is 

 quite possible that at one time the lower and higher plains may have been 

 continuous, but were subsequently separated by a depression of the region 

 to the northeast. In other words, the escarpment whicli limits the Jino- 

 tepe plateau to the north and east may i)Ossibly mark the line of a rather 

 recent fault. The escarpment has been deepl}^ scored by stream channels, 

 so that it does not now liave the characteristic form of a recent fault 

 scarp, but the character of the materials of which it is composed is such 

 that it would be rapidly modified, and so retain its original form but a 

 short time. 



Climate 



general character 



The climatic conditions prevailing in this region have so direct a bear- 

 ing on its geolog}^ and physiography that a brief statement of the more 

 important ch^iracteristics of the climate is essential. Lying only 10 de- 



