Hkrshkv.] 



Isthmus of Panama. 



265 



ORIGIN OF ISTHMIAN STREAM COURSES. 



The courses of nearly all the streams of the central portion of 

 the Isthmus of Panama are governed primarily by the general 

 slope of the dissected peneplains. They rise at the axes of the 

 great structural arches and flow directly down the slopes of the 

 tilted peneplains into the sea. Several small streams in the foot- 

 hills north of Santiago may be exceptions, as their courses seem to 

 have been determined in part by the northwest to southeast strike 

 of the strata, but, in general, I believe the drainage to be inde- 

 pendent of the rock structure. Certainly there has been a radical 

 readjustment since the Eocene volcanic period, and this seems to 

 have been accomplished mainly during the inception of uplift of 

 the Tertiary peneplain. The stream courses seem to have been 

 brought under some straightening process which may have been 

 the rapidity of tilting of the peneplain. 



In the Montijo basin the drainage has been reversed through 

 the elevation of the Cordillera de Veraguas and the submergence of 

 the "old land" on the south. Most of the rivers on the western 

 side of the Peninsula of Azuero flow down out of the mountains 

 toward the northwest and they may be relics of the drainage sys- 

 tem in the basin before the reversal. Just when that occurred I 

 could not determine. 



PLEISTOCENE OSCILLATIONS OF THE SOUTHWEST COAST OF NORTH 



AMERICA. 



The same evidences of a geologically very recent depression of 

 the coastal lands are found all along the Pacific side of the conti- 

 nent as far north as the Bay of San Francisco. Prof. A. C. Lawson* 

 and Dr. H. W. Fairbanks! have recently discussed the subsidence 

 on the Californian coast. Here the recent submergence appears to 

 have been rather local in character. The late Pliocene and early 

 Pleistocene submergence of the southern California coast, marked 

 by raised shore-lines, did not extend as far as the Isthmus of Pan- 

 ama, and I hardly think was represented on the coast of southern 

 Mexico and Central America. 



* Bulletin of the Department of Geology, University of California, Vol. I, 

 No. 4, December, 1893. 



tAmerican Geologist, Vol. XX, No. 4, October, 1897. 



