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



rate residual outline of the former oives place to the even constructional 

 slope of the latter. A single hreak occurs in this continuous line of 

 hills. This is the gap hetvveen the waters of the Rio Lajas and of the 

 Rio Grande. Here the level plain bordering the lake extends entirely 

 tlirougii the range of hills, forming a low, ))road gap whose summit is 

 but 50 feet above the lake. 



The manner in which this single low gap was formed is described at 

 some length in a later part of this paper, where the recent geological 

 history of the region is given. It may be stated here, however, that the 

 gap is the product of the familiar process of stream capture. Owing to 

 the decided advantages possessed bythe streams flowing directly to the 

 Pacific over those flowing eastward, at first to the bay of Nicaragua and 

 afterward to the lake, the former were able to cut back through the 

 divide into the drainage area of the latter arid to divert their head- 

 waters. In this way an eastward-flowing stream originally occuj^ying 

 the position of the Tola, the upper Rio Grande, the Guiscoyol, and the 

 Lajas was beheaded, and the drainage of a large part of its basin was 

 diverted to the Pacific. The deserted valley of this stream forms the 

 low gap through which the canal route is located. It is so broad and 

 level that accurate instrumental work is required to determine tlie actual 

 summit of the Continental divide. 



The Pacific coast in the southern part of this region is formed by alter- 

 nating short strips of sandy beach and bold, rocky promontories. The 

 stretches of beach are formed by the silting up of deeply cut valleys, and 

 the promontories by the truncated points of ridges which extend down 

 to the coast between the valle3^s. To the northward of Brito the pro- 

 posed western terminus of the canal, at the mouth of the Rio Grande, 

 the hills are farther inland and fewer spurs reach the coast. A coastal 

 plain of some extent is here developed, increasing in width to the north 

 until it passes beneath the recent volcanic deposits which form the 

 Jinotepe plateau. 



This coastal plain probably at one time passed around the northern 

 end of the divide hills and was continuous with the Rivas plain to the 

 east. With the formation of the Jinotepe plateau the tuffs of wliich it 

 is composed buried this northern portion of the plain and piled up 

 against the end of the divide hills three or four hundred feet in thickness. 



Lake- Caribbean divide. — The much greater rainfall in the eastern por- 

 tion of the isthmus has given the Caribbean streams a decided advan- 

 tage, and they have pushed the divide westward probably some distance 

 from its original position. A few cases occur which clearly indicate 

 stream diversion. ■ The most striking of these is the upper portion of the 

 Rio Grande, which flows to the Caribbean north of Bluefields. This 



