150 
GEOLOG Y AND MINERALOGY. 
and greenstone rocks appear in the majority of cases to accom- 
pany the most vertical of the stratified rocks, and to have been, 
by their intrusion and appearance upon the surface, the elevating 
rock. This crystalline rock (granite), however, does appear in a 
few places : it is first met with at the Pacific shore, where it con- 
stitutes the Cerro Morro, San Diego, Huilotepec, and extending 
north to the city of Tehuantepec, where it appears in the hills 
of Dani Guivedchi and Dani-lieza. These small hills, which are 
not above 500 feet above the plain, or 550 feet above the sea, 
are the eastern spur of a small granite range (the Cerro Yerde), 
which passes in a westerly direction through Oaxaca towards the 
Rio Yerde. In stretching west they become more elevated. 
The scattered hills which lie along the shore between this 
range and the lagoons, such as Malamamlaif, and a few of lesser 
note, are the most depressed points of this chain. Granite forms 
some of the higher summits in the mountain districts north and 
northeast of Cerro Atravesado. It is also met with to a small 
extent in the low ground leading from the Portilla de Tarifa to 
La Yenta cle Chicapa, along the Arroyo del Tolistoco. This 
rock is not met with in place in any other part of the Isthmus : 
drift and pebbles of it are found in the channel of the Coatza- 
coalcos River, carried down from the Sierra by the mountain 
torrents, showing its existence in that range. 
With one exception,- all the islands of the upper lagoon, and 
the hills which lie between it and the lower lagoon, are of green- 
stone, sienitic greenstone, with other metamorphic rocks, such, 
as porphyry and hypersthene. The greenstone there appears to 
have cut through the granitiform rocks and their contents, and 
the constituents of both seem to have become fused together. 
The island of Mitiac-xocuan in the upper lagoon, the Cerro de 
Buena Yista, and the range of hills between the Espantaperros 
and the Niltepec rivers north of the lagoons, as well as the 
scattered hills east of this range, are of porphyry, with a basis 
of jasper and claystone. These metamorphic rocks, produced by 
the action of the greenstone protrusions, appear to underlie the 
whole of the Pacific plains north of a line drawn from Tehuante- 
pec to Xocuapa, and extending to the Yenta de Chicapa. The 
northern and southern slopes of the eastern Cerro Prieto, and 
