HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 239 
RÉSUMÉ op THE GEOLOGICAL Composition OF THE TROPICAL MAINLAND. 
IN order to arrive at any conclusion concerning the correctness of 
these deductions it is necessary to make a brief review of what is known 
of the geologie composition of the adjacent Central and South American 
provinces. 
Incompleteness of Exploration. — There are many gaps in our knowl- 
edge of the geologie history of the Central American region. Several 
areas must be thoroughly explored before final conclusions can be made. 
The west coast from Lower California to the equator has hardly been 
touched upon by any explorer. We know nothing of the structure, pale- 
ontology, or source of sediments in this region. That portion of the 
northwest corner of South America lying west of the Rio Atrato, be- 
tween the northern terminus of the Andes and the Pacific, is still terra 
incognita. 
The detailed work of Sapper in Guatemala," Chiapas, and Yucatan,’ 
the study of the San Salvadorian volcanoes by Dollfuss and Monte Ser- 
ratt, Gabb's exploration of Costa Rica, the detailed sections herewith 
presented by the writer, and the explorations of Sievers è and Karsten 4 
along the Colombian and Venezuelan coasts, are the chief contributions 
to the geology of the mainlands of the western and southern perimeter 
of the Caribbean. 
Old. Granitic Rocks in the Caribbean Region." — From the Rio Grande 
border of the United States to the end of the Cordilleras in Southern 
Mexico, known as the great “ Abfall? of the plateau, no rocks of positive 
Archean or Paleozoic age are exposed, although they have been fre- 
1 Grundzuge der Physikalisehen Geographie von Guatemala. Von Dr. Carl 
Sapper. Dr. A. Petermann's Mitteilungen, 1894. 
2 Sobre la Geográfica y Física Geologia de la Península de Yucatan. México, 
1896. 
3 Sievers, Petermann's Mitteilungen, 1896, Bonn, Vol. XLII., Part 6. 
4 Karsten, Géologie de l’ Ancienne Colombie, Boliviarienne, etc., Berlin, 1886. 
5 The reader of all Tropical American literature must bear in mind that many 
rocks are called granites by writers inexperienced in mineralogy which are not 
granites at all, but either granulites, syenites, diorites, or light colored igneous rocks 
having a superficial resemblance to granites. The writer always submits his col- 
lection of igneous rocks to mineralogic experts for determination, but it has been 
the custom of many not to do this, and hence the misuse of the term “ granite " 
as above stated, Many of the light colored false granites belong to classes of 
'Tertiary or Post-Tertiary rocks which the writer has observed from El Paso, 
Texas, southward to Colombia and in the West Indies, upon which studies of 
future observers will throw much light. 
