120 bulletin: museum of comparative zoology. 



have been denuded. These beds appear to ha\e been the same 

 Cretaceous and Tertiary strata which occur in patches along the coast 

 in dislocated positions. The borders of the Coastal Cordillera are 

 determined by faults so that the Cordillera is itself a long relatively 

 uplifted fault-block or "horst." Relative to it, the western side of 

 the Longitudinal Valley on the east has sunk as has also the Pacific 

 Ocean border on the west. It is along the fault-zone on the west, 

 partly if not altogether, that the epicenters of great Chilean earth- 

 quakes- mainly he as was the case with that of Coquiml)o in ] 906, 

 which destroyed Valparaiso and neighboring towns. 



It follows from this structure of the west coast of South America 

 throughout the greater part of Chile that the earthquakes which take 

 place along the faulted edges of the Coastal Cordillera arise from the 

 movements of this horst and are not directly connected with the 

 folded chain of the Andes on the east. Accepting as I think we must 

 the essential accuracy of Darwin's and Fitzroy's obser\-ations upon the 

 elevation of the coast about Concepcion at the time of the earthquake 

 of 1835, it does not follow, however, as Darwin thought, that the 

 Andes were simultaneously elevated. 



In the Valparaiso earthquake of 1906 Professor Steffen has found 

 evidence of a local elevation amounting to half a meter on the western 

 border of the Cordillera at that place; but it is a question Avhether the 

 elevations which take place at the time of these earthquakes are 

 permanent. At Concepcion it is the opinion that a subsidence follows 

 the uplift of the coast. Nevertheless the horst stands as an indubita- 

 ble block of evidence of uplift in relation to the ocean bottom and' the 

 Longitudinal Valley. 



The date of beginning of the faults which bound the horst of 

 the Coastal Cordillera is Post-Eocene and apparently late Tertiary. 

 Whether movement has taken place in modern times along the fault 

 on the east I am not able to say. The Pleistocene gravels at San 

 Rosendo abut against the horst without signs of disturbance; and the 

 rivers, such as the Bio Bio, Calle Calle, and JNIaule, have cut channels 

 across it on their way to the sea from a superposed position. Neither 

 the Rio Bio Bio nor the Calle Calle, through whose valleys I passed in 

 traversing the width of the horst, display rapids such as might be due 

 to recent uplift at a greater rate than that of the cutting power of 

 these streams to maintain a free grade to the sea. 



It seems most probable that the eastern fault bounding the Longi- 

 tudinal Valley is of early Pleistocene or late Pliocene date, and that 

 the present valley is due to erosion of the lavas and sediments pan 

 passu with the cutting of the gorges. 



