222 bulletin: museum of compaeative zoology. 



fourth, the Atlantic declivity, extending from the summit of Turialba 

 to the waters of the Caribbean near Port Limon, a distance of only 40 

 miles, in which distance there is a rapid descent of nearly 12,000 feet. 

 Bordering this is a narrow coastal plain. 



The Peninsula of Nicoya. — This peninsula is dominated by low 

 mountains which seem in direct strike with those of the Salsipuedes 

 Peninsula and Coiba Island, but the geologic problems of this frag- 

 mentary continental section will not be completely solved until its 

 structure is studied in detail. In the appendices of this report will be 

 found a few supplementary remarks on this interesting region from 

 Mr. Sjogren. 



As viewed from the sea, the peninsula of Nicoya has the same charac- 

 ter of hilly topography as the adjacent mainland. Its rocks are largely 

 made up of silicates, quartzites in which greenish colors prevail, and 

 the ancient Isthmian eruptives. I studied the collection of gravels 

 brought down from this peninsula into the Bay of Dulce. They all 

 indicate that this is the prevalent hard Tock of the region. Gabb states 

 that lignite-bearing clays are reported from the peninsula, but we have 

 not a single line of evidence from any observer concerning its geologic 

 position or relations. 



The green quartzites are apparently the remnant of the oldest rocks 

 exposed in Costa Rica, unless it be the Siquieres granite. On the 

 eastern declivity, where the rapid fall of the Itevantazon cuts down 

 within a short distance nearly 4,000 feet, and below, making the 

 lowest exposures, these rocks are again exposed and brought down as 

 gravel, showing that they must underlie the vast accumulation above 

 them. 



The San Mateo Peneplain. — The region between Punta Arenas and 

 Aguacate consists almost exclusively of the older basic igneous rocks 

 and their debris. These rocks, as has previously been shown by Attwood 

 and Huddleston, 1 are nearly all augite andesites. The country from the 

 coast to the foot of the Aguacate range is a typical peneplain which has 

 been elevated to an average height of about 800 feet. This has been 

 cut into numerous vertical chasms by the newer drainage which has 

 developed upon it. 



No sedimentary strata or beds of massive igneous rocks of any kind 

 are seen along this region. The sole geologic formation consists of an 



1 On the Geology of a Part of Costa Rica, by George Attwood, Esq.; and 

 Report on some Rock Specimens collected by George Attwood, Esq., by W. II. 

 Huddleston. 





