hill: geology of the ISTHMUS OF PANAMA. 239 



Resume of 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 geologic composition of the adjacent Central and South American 

 provinces. 



Incompleteness of Exploration. — There are many gaps in our knowl- 

 edge of the geologic 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, 1 Chiapas, and Yucatan, 2 

 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 3 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 Rods 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 

 Archaean or Paleozoic age are exposed, although they have been fre- 



1 Grundzuge der Physikalischen Geographie von Guatemala. Von Dr. Carl 

 Sapper. Dr. A. Petermann's Mitteilungen, 1894. 



2 Sobre la Geografiea y Fisica Geologia de la Peninsula de Yucatan. Mexico, 

 1896. 



3 Sievers, Petermann's Mitteilungen, 1896, Bonn, Vol. XLIL, Part 6. 



4 Karsten, Geologie 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 whicli 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. 



