﻿16 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



topography and the prevailing winds and the resulting variations 

 in rainfall. 



The climate is now moist tropical, modified by the nearness of 

 the two oceans, and there is therefore but slight diurnal or annual 

 variations in temperature. So far as information is available re- 

 garding the conditions during the Tertiary, there is no evidence that 

 can be deduced from the fossil flora or the geographical history of 

 the region to indicate that the climate was very different from what 

 it is now at any time during the Tertiary, unless we are prepared to 

 assent to enormous changes in the altitude of the land, for which 

 the data does not seem to be adequate. 



The prevailing winds now come from the northeast, and as the 

 divide is near the Pacific Coast the major part of the Isthmus north 

 of this low divide has a heavy rainfall, as, for instance, 170 inches 

 dt Porto Bello and 129 at Colon, as compared with 90 inches at 

 Culebra or 71 inches at Ancon. There are two seasons — a short 

 relatively dry season extending from January to April and a long 

 and relatively wet season the balance of the year with the maximum 

 of precipitation from September to December. Before the clearing 

 of the French Canal Company forests covered six-tenths of the 

 Isthmus, the remainder being broken forests and savannas. Ever- 

 green tropical rain-forests of mixed angiosperms covered the entire 

 northern watershed and part of the Darien region on the south side. 

 Some of the forests of the southern watershed are what are known 

 as monsoon forests, with many deciduous species, and at high alti- 

 tudes there may be more gregarious types of forest as, for example, 

 the oak forests which are so striking a feature in the uplands of 

 Central America as you proceed to the northwest. 



The shores are skirted with dunes abounding in Leguminosae and 

 Euphorbiaceae with Coco palms and Hippomane. Low shores and 

 tidal inlets are covered with mangrove swamps with Rhizophora, 

 Avicennia, Conocarpus, etc. Less saline coastal marshes are covered 

 with Acrostichum, Crescentia, or Paritium thickets. The evergreen 

 forest is composed chiefly of species of Sterculiaceae, Tiliaceae, and 

 Mimosaceae, Euphorbiaceae, Anacardiaceae, Rubiaceae, Myrtaceae, 

 and Melastomataceae, with small palms like Chamaedorea, Trithri- 

 nax, and Bactris. 



CORRELATION. 



The fossil flora described in the present report is too limited for 

 purposes of exact correlation, which may be expected to be settled 

 by the marine faunas present at most horizons in the Isthmian region. 

 Regarding the plants in the various formational units recognized in 

 the Canal Zone by MacDonald a glance at the accompanying table 

 of distribution will show that from the oldest (Bohio) to the young- 



