E. Howe — Geology of the Isthmus of Panama. 213 



at many places the information to be obtained from snrface 

 outcrops was augmented by a great number of boring records 

 and samples. 



Special thanks are due Mr. John F. Stevens, under whom 

 as chief engineer the work was done, for many facilities placed 

 at my disposal. I am also greatly indebted to Dr. William H. 

 Dall for his kindness in looking over my collections and for 

 many valuable suggestions. 



Preliminary Statement. 



The Isthmus of Panama, where it is to be crossed by the 

 canal, consists of sediments and pyroclastics of Tertiary age 

 that rest on an eroded surface of andesitic breccias and associ- 

 ated lava flows, all of which have been intruded at numerous 

 places, probably in Miocene time, by dikes and large cross- 

 cutting masses of andesite or basalt. During the period of 

 intrusion, or immediately after it, the region was uplifted, and 

 the cycle of erosion thus inaugurated continued to late matu- 

 rity or old age. Before the completion of the cycle, however, 

 another upward movement occurred accompanied by warping 

 or a gentle medial doming. As a result of the continued ero- 

 sion the basal igneous mass was exposed in the interior and a 

 sub-mature topography developed in regions of harder rocks, 

 while near both coasts less resistant sedimentary beds favored 

 more active erosion and conditions of greater maturity pre- 

 vailed. A third uplift caused the streams to entrench them- 

 selves in their old or mature valleys, but before this last cycle 

 had advanced beyond its youth, a gradual subsidence began 

 that continued until the young valleys were aggraded and the 

 mature valleys of the second cycle drowned for short distances 

 back from the coasts. A final slight upward movement has 

 elevated the estuarine deposits, formed during the period of 

 depression, a few feet above sea level. 



Obispo Breccias. 



The igneous complex that appears to underlie all of the 

 other formations of the isthmian region consists largely of 

 andesitic tuffs and breccias ; it has been given the name Obispo 

 on account of its characteristic exposures at Bas Obispo, the 

 northern end of the Culebra cut. From Bas Obispo south- 

 ward to a point between Empire and Culebra the breccias 

 have been well exposed by excavation, but at the time of exam- 

 ination the zone of surface alteration had not been passed, so 

 that although the rocks show their pyroclastic nature the petro- 

 graphical character of the fragments and the matrix is not 

 readily determinable. Fresher material was collected on the 



