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



adding to its drainage area and hence to its volume by encroaching on 

 the basing of its neighbors. There is no evidence from the arrangement 

 of the drainage in tliis region that the Rio Grande has ever lost any ter- 

 ritory in this way. 



2. The former volume of the Rio Grande miglit have been greater by 

 reason of different climatic conditions which at some former time gave 

 the region a greater rainfall than it now has. There is no direct evidence 

 in favor of this hypothesis. So far as known, there is no evidence what- 

 ever that the rainfall has ever been greater in this region than it is at 

 the present time. On the contrary, if a greater rainfall has been the 

 cause of the old valley, this condition would have been general in its 

 effects, and all the streams of the region would show the same evidence 

 of greater volume in the past. So far as known, however, the Rio Grande 

 is exceptional in this respect. 



3. The third possible explanation is that the lake may have found an 

 outlet for a short time by way of the Rio Grande valley. As pointed 

 out above, the lake rose behind the barrier formed of volcanic ejecta 

 until the level of the impounded waters reached the lowest gap in the 

 Continental divide, where they spilled over and escaped by way of a 

 river channel leading eastward to the Caribbean sea. Now the material 

 forming the gap in the divide must have been residual clay and deeply 

 weathered rock, material which would be rather readily removed by the 

 corrasion of the escaping waters ; also a study of the present river gorge 

 where the Continental divide formerly existed shows that the channel 

 has here been considerably lowered. It does not seem at all improb- 

 able, therefore, that the lake for a short time may have been 50 or more 

 feet higher than now with refrence to the surrounding countr}^ ; but if 

 it were raised 50 feet, its waters would escape by the Lajas-Grande gap 

 westward to the Pacific. It seems possible that when the waters of the 

 lake were first raised by the growing barrier to the northwest, they found 

 two gaps at approximately the same altitude, and for a time escaped in 

 part east to the Atlantic and in part west to the Pacific. Active corra- 

 sion of the two outlets began at once. The gorge of the Rio Grande was 

 excavated, but the gap in the main divide in the east was at first in less 

 resistant material, and was consequently cut down the more rapidl3\ 

 By the time hard rock was reached in this gap the waters had been 

 entirely withdrawn from the western outlet. The eastward tilting of the 

 region west of the lake may have continued well into this period, and 

 have been in some measure instrumental in finally turning the outlet to 

 the east. 



It is possible that at first the gap in the main divide to the east was 

 so much higher than the one to the west that all the water escaped by 



