298 C. W. HAYES GEOLOGY OF NICARAGUA CANAL UOUTE 



bottom of the canal, so that the excavation necessary to secure the re- 

 quired dei)th will be entirely in alluvium. 



ResUlKdl Jiills.— The third group of topographic forms which character- 

 ize the old land area embraces the hills rising distinctly above the present 

 tops of the lower hills and representing portions of the surface never re- 

 duced to the level of the old pene})lain. The summits of these hills are 

 entirely different from, the dissected remnants of the peneplain above 

 described. The crests are always sharp and serrate, with no uniformity 

 whatever in their altitudes. 



The hills of the Eastern divide lying between the basins of the Deseado 

 and San Francisco form a characteristic group belonging to this class. 



Their slopes are extremely steep and their sides are furrowed by sharp 

 V-shaped ravines. Around their base are remnants of the old plain 

 above which they formerly rose, now appearing as rounded hills with 

 uniform summits. Long spurs radiate from the central mass of the 

 Eastern Divide hills and reach the San Juan river at several points, 

 forming the high ridges at Sarapiqui, Tamborcito, Tamborgrande, and 

 San Francisco. Another i)rominent group of hills belonging to this 

 series occurs at the junction of the San Juan and San Carlos. These 

 have a form similar to that of the Eastern Divide hills, but the group is 

 somewhat smaller. The upper slopes are extremely steep and the sides 

 are deeply gullied, while the summit as seen from either side presents a 

 sharply serrate outline. The altitude of the San Carlos hills is about 

 1,200 feet. These isolated groups of high hills occur with increasing 

 frequency toward the line formerly occupied b}^ the Continental divide, 

 which probabl}'' crossed the present valley of the San Juan in the vicinity 

 of Castillo. West of this line they decrease in frequency and hight to 

 the lake. 



The residual hills which rise above the peneplain of the Nicaraguan 

 depression increase in hight and numbers toward the north, finally'' 

 merging with the mountains of northern Nicaragua, where they reach 

 elevations from 6,000 to 7,000 feet above tide. Comparativel}' little is 

 known of any portion of this region except its western margin. The 

 eastern part is covered with a dense tropical forest, is almost entirely 

 without settlement, and has been only partiall\^ explored. The divide 

 between the lake and Caribbean drainage passes some distance to the 

 westward of the axis of the isthmus, being approximate! }'■ parallel with 

 the Pacific coast northwestward to the IMatagalpa river, where it makes 

 an abrupt bend to the eastward, passing around the basin of that stream. 

 This region between the lake and the Caribbean may be described as a 

 deeply dissected upland. During Tertiar}'^ time it was doubtless the 

 locus of intense volcanic activity, but subsequent erosion has entirely 



