PHYSIOGRAPHY OF NICARAGUA CANAL ROUTE 241 



irregularly rounded form seen in Madera and Zapatero. In 

 some cs w the conical summit is destroyed, not by the slower 

 process &w erosion, but by an explosive eruption. The result is 

 an irregular, jagged truncation of the cone, usually with a de- 

 pression in the center occupied by a lake. This form is seen in 

 Mombacho and Coseguina. There is a tradition that the former 

 was once a conical peak, and, as is well known, the summit of 

 Coseguina was blown off by an explosive eruption in 3843, the 

 most violent recorded eruption until surpassed by that of Kra- 

 katoa in 1883. 



These volcanoes rise from a level plain which stretches from 

 the head of Lake Nicaragua northwestward to the Gulf of Fon- 

 seca. It is composed wholly of fragmental volcanic materials, 

 which reached their present position probably in the form of 

 mud flows. The materials are not sorted, though successive 

 layers are separated by distinct planes. In a quarry near Ma- 

 nagua human tracks have been found in this material, showing 

 that it is very recent, and also affording some indication of the 

 physical conditions under which it was deposited. To the 

 southwest of this plain and separated from it by a rather steep 

 escarpment is a plateau which slopes somewhat gradually from 

 an altitude of about 1,200 feet along its inner margin southwest- 

 ward to the Pacific coast. This plateau is composed of exactly 

 the same materials as the lower plain to the east, and it appears 

 probable that the two surfaces Were once continuous, but have 

 recently been separated by a fault. 



From all the evidence thus far obtained, it appears probable 

 that during early Tertiary time the waters of the Atlantic and 

 Pacific had free intercommunication across this portion of the 

 isthmus. During that time sedimentary rocks were accumulated 

 over a considerable portion of the isthmus and great masses of 

 volcanic rocks were poured out upon them. In middle Tertiary 

 time the region was elevated above sea-level, and there is no evi- 

 dence that it has at any time since been depressed so as to give 

 free communication between the two oceans. The elevation of 

 the region was followed by a long period of erosion, during which 

 its surface was reduced to a broadly undulating plain. The 

 main divide was then near the axis of the isthmus, and from this 

 divide streams flowed in opposite directions — eastward to the 

 Caribbean and westward to the bay which indented the Pacific 

 coast. After the surface of the country had been considerably 

 reduced across this narrowest portion of the isthmus the region 



