202 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the period of basic igneous eruptivity could possibly have been deposited 

 so free from its characteristic debris. Besides, there are no known Post- 

 Cretaceous areas of quartzitic rocks now exposed throughout the region 

 from which such acidic formations could have been derived in Tertiary 

 or later time. Hence I am inclined to think that the Panama formation 

 represents a remnant of the older Pre-Tertiary rocks of the Isthmian 

 region. It is an interesting fact, also, that both Garella and Maack 

 considered these rocks described by them as conglomerates and sand- 

 stones of Pre-Cretaceous age. 1 



The Panama formation certainly has, as Maack and Garella have pre- 

 viously stated, the colors and stratigraphic appearance of the older 

 Mesozoic sandstones, both of Europe and the Western United States, 

 but age conclusions cannot be drawn from snch lithologic resemblances. 



The Panama Bench or Base Levelled Plain. — Around the base of 

 Cerro Ancpu, bordering Panama Bay, is a low hilly bench standing at 

 an average height of about 75 feet above the level of the sea and the 

 swamps, and forming a low shoulder between it and the sea. The 

 rolling summit of this bench, cut by numerous drainage lines which 

 cross it, is traceable between the mountains and the sea eastward from 

 Panama toward the mouth of the Rio Grande and the canal, the city 

 proper and outlying cemeteries being built upon it. Beyond the Rio 

 Grande it extends eastward at intervals for twenty or thirty miles, and 

 may be traced a still greater distance. From its irregular summit to 

 the waters of the bay there is in places a sharp escarpment or bluff 

 which is being undermined by the waves at high tide, and its debris dis- 

 tributed over the surf line of the bay. The extreme seaward point of 

 Panama, upon which the military post is located, is of this nature. 



This bench is an old base levelled plain, reduced to its present topo- 

 graphic aspect in past geologic time, when the coast was much lower 

 than at present, and taken in connection with the lower Rio Grande 

 swamp level presents a remarkable historic analogy to the corresponding 

 features of the Monkey Hill base levelled plain and Mindi swamp level 

 of the Caribbean side, which, when taken together, testify that the Isth- 

 mus has most certainly participated in certain uniform (Epeirogenic) 

 movements, in addition to the disturbances of volcanic orogenic (wrin- 

 kling) character. It will he seen as we proceed that the Panama base 

 level is a persistent feature in the Costa Rican and Panama region, and 

 is well developed upon the islands both of Panama Bay and the Pacific, 

 and the adjacent coast of the mainland at least as far as Puuta Arenas 



in Costa Rica. 



1 Op. cit., page 164. 



