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SCIENCE 



[N. S. Vol. XXVI. No. 657 



non-glacial, on the other. It shows the Tose- 

 mite Valley together with a host of other 

 glaciated canyons and valleys; but besides 

 giving us an inkling of its unusual nature, 

 tells us nothing except that it has glacial char- 

 acteristics. The new detail map, on the other 

 hand, depicts the Tosemite Valley, not merely 

 as a glaciated canyon, hut as a glaciated 

 canyon in a region of unusual rock structure. 

 It is a map giving index forms of differential 

 erosion and cliff recession, and brings out the 

 fact that the aberrant character of the Yose- 

 mite topography is intimately linlied with the 

 structural vagaries peculiar to the rocks of the 

 Yosemite region. 



Geology of the Canal Zone: Ernest Howe. 



Passing from the Atlantic to the Pacific, the 

 line of the Panama Canal traverses three well- 

 defined topographic divisions. The first is 

 that of the lower valley of the Chagres and 

 includes the swampy lowlands that extend 

 from Limon Bay nearly to Bohio. This divi- 

 sion ends at San Pablo, about six miles below 

 the point where the Rio Obispo enters the 

 Chagres. The second division is that of the 

 summit region and, extends from San Pablo 

 to Pedro Miguel, while the third lies between 

 Pedro Miguel and La Boca, and like the first 

 is low and swampy. The relation of these 

 divisions to one another is well shown by the 

 map of the proposed lock canal, the two re- 

 gions of low relief being marked by artificial 

 lakes while the summit region is traversed 

 by the canal cut. 



Although difiicult to decipher, on account 

 of the deep covering of red clay and vegeta- 

 tion, the geology is itself quite simple. The 

 oldest rocks of the region are andesitic brec- 

 cias that occur in the central area between 

 Mamei and Empire and again in the higher 

 hills northeast of Panama. Northward sedi- 

 mentary rocks occur resting on the older 

 igneous mass and gently inclined toward the 

 Caribbean, so that in passing from the in- 

 terior toward Colon successively younger beds 

 are encountered. They are well stratified and 

 contain abundant fossils of early Tertiary age. 

 Nearly the same conditions prevail on the 

 Pacific side. On both sides are stratified de- 



posits of acid pyroclastics, the most conspicu- 

 ous being near the city of Panama. Dikes 

 and large cross-cutting masses of augite-ande- 

 site or basalt have invaded all of the older 

 sedimentary rocks, and occur in great abun- 

 dance in the southern and central parts of 

 the zone; they represent the last phase of ac- 

 tive vulcanism in the region. 



With the exception of beds of heavy con- 

 glomerate in the vicinity of Bohio, all of the 

 sedimentary rocks of the Isthmus consist of 

 argillaceous sandstones, greensands or fine 

 sandy shales; limestones, except high up the 

 Chagres River and in the neighborhood of 

 Empire, are unknown. The rocks are well 

 bedded with moderate northerly dips north of 

 the central region, while the sediments south 

 of the Culebr^ Cut are inclined in the oppo- 

 site direction. The oldest beds are Eocene, 

 while the youngest, found near Colon, are late 

 Oligocene. The complete section is preserved 

 only on the northern flanks of the isthmus. 

 Excavation for the locks at Gatun will be in 

 argillaceous sandstones of the early Oligocene 

 throughout, and actual tests on the spot have 

 shown that they are capable of withstanding 

 pressures many times greater than those to 

 which they will be subjected by the lock walls. 

 The earth dam that will be thrown across the 

 valley at Gatun will rest in part upon alluvial 

 material filling a deep gorge cut by the 

 Chagres in Pleistocene time. For more than 

 100 feet below the surface this alluvium con- 

 sists very largely of fine blue clay and silt and 

 will be entirely impervious to water. Foun- 

 dations for the locks at Pedro Miguel will be 

 in the sandy shales and sandstones of the 

 Culebra beds of Eocene age, while the ma- 

 terial at the lock site at Sosa at the Pacific 

 end of the canal is massive augite-andesite. 

 The dams at La Boca and Sosa will be of 

 earth and will rest upon alluvial clays of the 

 lower Rio Grande valley. 



Recent Changes in the Ice Fields of Glacier 

 Bay, Alaska: Charles Will Wright. 

 Mr. Wright described in some detail the 

 recent remarkable general recession of the 

 glaciers in Glacier Bay, Alaska. A summary 

 of the geologic history of Muir Glacier was 



