118 Reviews — Geology of the Panama Canal. 



sheet 200 feet thick, and there is no indication that the stream ever 

 followed any other course. The best examples of river erosion are to 

 be seen in the gorges of the Malutis, but 1 have never had the good 

 fortune to visit them. The scenery of these parts of the country 

 is remarkably grand in places, and well merits the title bestowed upon 

 it of the Switzerland of South Africa. 



It E -V I E -V^S. 



I. — Geology or the Panama. Canal. 

 ri"^0 the joxirnal entitled "Economic Geology, with which is incor- 

 X porated the American Geologist" (vol. ii, Oct. -Nov., 1907), 

 Mr. Ernest Howe contributes an essay on "Isthmian Geology and the 

 Panama Canal." He observes that the widespread decomposition to 

 which all the rocks have been subjected, and the thick mantle of soil 

 and vegetation, offer great obstacles to the stud}' of the geology of the 

 region ; but he was fortunately able to examine numerous artificial 

 excavations and records of borings. The oldest rocks along the Canal 

 route, andesitic tuffs and breccias with associated flows, occur in the 

 central and southern portions. Resting upon these on the Atlantic or 

 northern side of the Isthmus are fossiliferous conglomerates, sandstones, 

 and shales which dip at low angles toward the Caribbean Sea, so that 

 in passing from the interior to Colon successively younger beds are 

 encountered, the oldest carrying an Eocene fauna, and the youngest 

 late Oligocene, according to Mr. W. H. Dall. On the Pacific or 

 southern slope the Eocene strata extend for some distance and 

 disappear beneath beds of acid pyroclastic rocks, whicli in turn are to 

 be traced almost continuously to the shore near the Pacific end of the 

 Canal. A neon Hill, which dominates the city of Panama, consists of 

 massive rhyolite porphyry, that is believed to mark the site of 

 a volcanic vent, from which the acid tuffs and breccias of the neigh- 

 bourhood were erupted. Tuffs of nearly the same composition are 

 found on the Atlantic side in the vicinity of San Pablo and Tabernilla, 

 and similar rocks occur at Gatun, interbedded with fossiliferous 

 Oligocene sediments. The deposits on the Pacific side are believed to 

 be contemporaneous with these. Cutting all these earlier Tertiaiy 

 rocks are irregular dykes and stock-like masses of pyroxene-andesite 

 and basalt, which are considei-ed to be of Miocene age. 



The general structure of the Canal route is therefore a broad flat 

 anticline, having a nearly east-west axis ; but there are many small 

 folds and faults, and evidences of local vmconformity, which show that 

 differential movements of the land took place at intervals from the 

 Eocene period onward. 



The author deals with the origin of the present topographic features 

 that have resulted from the erosion and deposition that attended three 

 well-marked epeirogenic movements ; and in connection with them 

 a notable feature, discovered and located entirely by means of borings, 

 is a buried Pleistocene valley, that has afforded not only great scientific 

 interest, but also the most important problem with which the engineers 

 of the Canal have had to deal. 



