202 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
the period of basic igneous eruptivity could possibly have been deposited 
so free from its characteristic débris. Besides, there are no known Post- 
Cretaceous areas of guartzitic rocks now exposed throughout the region 
from which such acidic formations could have been derived in Tertiary 
orlater 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.! 
The Panama formation certainly has, as Maack and Garella have рге- 
viously stated, the colors and stratigraphie appearance of the older 
Mesozoic sandstones, both of Europe and the Western United States, 
but age conclusions cannot be drawn from such lithologic resemblances. 
The Panama Bench or Base Levelled Plain. — Around the base of 
Cerro Angon, 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 débris dis- 
tributed over the surf line of the bay. Тһе 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 be 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 Punta Arenas 
in Costa Rica. 
1 Op. cit., page 164. 
