GEOLOGY AND MINERALOGY. 
151 
the flanks of the Oerro de la Cienega are made of a silicious 
slate, or greywacke, as it has been termed in Mr. Moro's report. 
None of these altered rocks are on the line of survey. These 
hills stand prominently up out of the sandy plain in which the 
river and city of Tehuantepec lie. The character of the granite 
is more hornbiendic than felspathic or micaceous, and in some 
positions it is a distinct sienitic rock. Along the shore, granite 
is not confined to the situation of a massive rock, but is found 
as an intruded mass, or as intersecting veins in greenstone. This 
may be seen in Natartiac and a few other islands in. the upper 
lagoon. It may be that this appearance is due to the greenstone 
rock, which, cutting its way through a granitic or sienitic rock, 
ruptured off fragments, and carried them onward with the cool- 
ing mass, depositing them at a higher point as the intruding 
mass cooled. The axis of most of the hills near the Pacific and 
upon the islands lies in a direction east and west, with the gentle 
slopes in a southerly direction. With the exception of these 
granitiform rocks, the plains on the Pacific side are undisturbed 
by any elevated range. This large tract of country is covered 
over in most places by an alluvial sand. Along the course of 
the rivers this covering passes into a clay bottom, more or less 
rich, depending on the source whence the river derived its mud 
and detritus, the overflow of which contributes so much to enrich 
the land. The depth of this clay is in many cases thirty feet, 
mixed in layers with gravel. 
The hilly ranges which obstruct the Isthmus are a few chains 
of low mountains that appear to be the terminating points of the 
sierras which lie on either side. They are the Cerros de la Hu- 
camaya, Prieto, Masahua, Masahuita, Espinosa, and the Piedra 
Parada. 
The Hucamaya range is a continuation of the Sierra ; and the 
Prieto, Masahuita, and Espinosa are southeasterly prolongations 
of the chain. The Masahua are a parallel range to these last. Be- 
tween these two runs the Torrentera de Masahua. No erupted 
rocks are conspicuous in these ranges ; they are chiefly made 
up of early secondary rocks (silurian), which are very metamor- 
phic in their physical character. The southern flank of the Ma- 
sahua is a sandstone or grit, partly calcareous and ferruginous. 
