STREAM DIVERSIONS EAST OP THE LAKE 301 



river heads in the high valley of Matagalpa, from which it flows south- 

 westward for 35 miles, approaching the Viejo within about 5 miles, being 

 separated from that stream by a level swampy plain. The Viejo flows 

 southwest to the upper end of lake Managua, and it is entirely probable 

 that the upper portion of the Rio Grande was formerly a tributary of the 

 Viejo. From the point where it approaches most nearly to the Viejo it 

 flows southward for a distance of 25 miles, and this southerly direction 

 is continued in a tributary which enters at that point. This portion of 

 the stream appears to have been at one time a part of the Malacapoya, 

 which enters the head of lake Nicaragua. From the point of nearest 

 approach to the Malacapoya the Rio Grande turns abruptly back to the 

 northeast, and for a distance of 30 miles is approximately parallel to its 

 upper course in the valley of Matagalpa. It appears highly probable 

 that the Rio Grande, by reason of the greater rainfall in the eastern part 

 of this region, pushed the divide westward until its headwaters inter- 

 cepted the upper portion of the Malacapoya. The same process was 

 continued and the extended headwaters effected another conquest, di- 

 verting a large tributary of the Viejo. The latter capture has been so 

 recent that the channel of the diverted stream has not been perceptibly 

 lowered, and a part of its waters in the wet season may still follow their 

 former course to the Viejo across the intervening swampy plain. A few 

 other cases of stream diversion are indicated by the character of the 

 present stream channels, but none of them are so striking or important 

 as that of the Rio Grande. 



Volcdiiic mountain ranges. — As indicated above, the southern margin of 

 the Nicaraguan depression is formed by the foothills of the Costa Rican 

 volcanic range. This range terminates to the northwestward in the prob- 

 ably extinct volcano Orosi. It contains a large number of volcanic peaks, 

 most of which are extinct and a few quiescent or moderately active. 

 These peaks have a striking linear arrangement and form two nearly 

 parallel lines of vents. The line terminating in Orosi extends southeast- 

 ward into Costa Rica, passing to tiie southward of a parallel range whose 

 northern peak is the volcano Turrialba. These two lines are about 10 

 miles apart, but their peaks are so high that their slopes merge and they' 

 form a single range. If the line connecting the northeastern series of 

 peaks were continued to the northwestward through the southern por- 

 tion of lake Nicaragua it would coincide very nearly with the line con- 

 necting the peaks of the Nicaraguan range. The latter range terminates 

 to the southward in the extinct volcano of Madera; thence it stretches 

 to the northwest, terminating in the volcano Coseguina, which occupies 

 a peninsula projecting into the gulf of Fonseca. Between these two ex- 

 treme peaks there is a large number of extinct, quiescent or active vol- 



XLIV— Bull. Geol. Soc. Am., Vol. 10, 1898 



