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



between the Atlantic and the Pacific oceans across this portion of the 

 isthmus, although it will be readily conceded that this conclusion is 

 merely an hypothesis. Sedimentary Formations have not as yet l)een 

 traced entirely across the isthmus, and there is no other direct evidence 

 by which this h3q3othesis can be proven. If, however, there had been 

 any land separating the two oceans, its rocks ought to be recognizable 

 at the present time as distinctly older than the Tertiary sediments or the 

 volcanic rocks which are intimately associated with them. As already 

 stated, no such older rocks are recognized in the region of the Nicaraguan 

 depression, and, although the volcanic activity whicli was contempora- 

 neous with the deposition of the sedimentary formations may have cut 

 off the communication between the two oceans early in Tertiary time, 

 it appears at least probable that at the beginning of that period, and i)er- 

 haps through the Oligocene, the sea had free access across the isthmus. 



The argument for the hypothesis of free communication across the 

 isthmus in Tertiary time has been very fully made by Hill.* A single 

 link, but a very important one, is wanting in the evidence obtained at 

 Panama. This missing link in the evidence is supplied by the discovery 

 of sedimentary beds on the Pacific coast of Nicaragua containing the 

 same fossils as the beds previously found on the Caribbean side of the 

 isthmus. Supplementing Hill's argument with this new evidence, there- 

 fore, the case seems to be definitely settled. 



The conditions which prevailed during the deposition of the sedimen- 

 tary rocks were somewhat shallow seas, with an abundant suppl}'' of 

 sediment alternating between sand and mud. The sediment appears to 

 have been chiefly derived not from a region underlain by deeply decayed 

 rocks, but rather from unconsolidated and recently ejected volcanic ma- 

 terial. The extremely coarse conglomerates which occur in the Brito 

 formation along the Pacific coast and on the southwest shore of lake 

 Nicaragua point to the proximity of active volcanoes. The coarser ma- 

 terial supplied by these volcanoes was transported but a short distance, 

 and shows the effect of only a moderate amount of wear. The finer 

 material was widely disseminated, and constitutes a very considerable 

 proportion of the sedimentary formations. These contain, however, a 

 certain proportion of clay, doubtless derived from the residual mantle 

 covering the older rocks which formed adjacent land areas. 



The conditions at certain points were favorable for the deposition of 

 limestone. Considerable lime is disseminated throughout the entire Brito 

 formation, and is segregated in marl}^ beds and in occasional lenses of 

 pure limestone. The volcanic activity not only furnished a large portion 



* Robert T. Hill : The geological History of the Isthmus of Panama and portions of Costa Rica. 

 Bull. Mus. Comp. Zool., vol. xxviii, 1898. 



