222 BULLETIN: MUSEUM OF COMPARATIVE 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 а 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 rock 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 guartzites 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 Revantazon 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 débris. These rocks, as has previously been shown by Attwood 
and Haddleston,! 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, Esg., by W. H. 
Huddleston. 
