240 BULLETIN ` MUSEUM OF COMPARATIVE ZOOLOGY. 
guently reported and may be present in the Western Sierra Madre. 
Time and again has the writer visited numerous alleged Archean local- 
ities in this region only to find that they were igneous rocks intrusivo 
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 Archean age. 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 He clearly 
shows that in this region granites, tale, 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 % 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-Archeean intrusion of granitoid 
rocks. 
The next outcrop of granitic rocks which may supposedly be Pre-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 débris 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 Beitrage zur Geologie und Paleontologie der Republik Mexico, Stuttgart, 
1891-93. 
? Grundzuge der Physikalisehen Geographie. Von Dr. Carl Sapper. Peter- 
mann's Mitteilungen, Erganzungsheft No. 113, Gotha, Justus Perthes, 1804. 
? The Naturalist in Nicaragua, by Thomas Belt, London, 1874, p. 259. 
