240 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



quently reported and may be present in the Western Sierra Madre. 

 Time and again has the writer visited numerous alleged Archsean local- 

 ities in this region only to find that they were igneous rocks intrusive 

 into a great sheath of Cretaceous sediments which mantles most of Mex- 

 ico north of latitude 18°. 



Along the 18th parallel in the State of Oaxaca low east and west 

 mountains begin to appear. These are composed of granites and gneisses 

 which, according to Felix and Lenk, are of Archsean age. 1 The general 

 strike of these mountain masses of Oaxaca is east and west, and not 

 northerly, as is the trend of the great North American and Andean Cor- 

 dilleras. These mountains mark the beginning of a remarkable series 

 of east and west orogenic axes succeeding one another in echelon parallel 

 arrangement across the Central American region. 



Between the 15th and 17th parallels, in the State of Chiapas, Mexico, 

 and the Republic of Guatemala, there is another group of east and west 

 trends composed of what are apparently Pre-Paleozoic granites. For- 

 tunately we have very definite information upon the geology of this 

 region, thanks to the recent publications of Dr. Carl Sapper.' 2 He clearly 

 shows that in this region granites, talc, and chloritic schists constitute 

 the stocks or massifs of ranges of older Paleozoic rocks, covered by a 

 Pre-Carboniferous limestone, presumably Silurian. 



Continuing eastward through Central America, Belt 3 has described 

 in Nicaragua a series of rocks which he refers to the Laurentian forma- 

 tion, consisting of " fundamental gneiss . . . covered by strata of much 

 more recent origin." His descriptions of these rocks, no doubt owing 

 to the difficulties of observation in the country, are somewhat incom- 

 plete, and it may prove that the folding of the schists and slates which 

 he describes is the product of a Post-Archaean intrusion of granitoid 

 rocks. 



The next outcrop of granitic rocks which may supposedly be Pie-Ter- 

 tiary or fundamental is in the River Siquieres, on the Atlantic slope of 

 Costa Rica, mentioned in our description of that country. Professor 

 Wolff's determination of the presence of granitic debris in the old Eocene 

 fossiliferous sedimentary rocks of this vicinity is an important point, as 

 it shows that granites, at least older than the Tertiary strata deposited 



1 Beitr'age zur Geologie und Paleontologie der Republik Mexico, Stuttgart, 

 1891-93. 



2 Grundzuge der Physikalischen Geographic Von Dr. Carl Sapper. Peter- 

 mann's Mitteilungen, ErganzangBheft No. 113, Gotha, Justus Perthes, 1894. 



8 The Naturalist in Nicaragua, by Thomas Belt, London, 1874, p. 259. 



