330 MEMORIAS DE LA SOCIEDAD CIENTIFICA 
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Opposite Zacatlancillo and also the Trinidad mine (K. 86) and back 
of the line of section are indicated extensive rhyolite dykes forming 
prominent mountains. 
Reaching the lower ground (K. 96) the breccias of the Miocene ex- 
tend beyond the Balsas river and are met with throughout the lower 
flat country except where concealed by the alluvial soil. The Nanchi 
and Aguila mountains are indicated in the back ground. These moun- 
tains are compound of the same or similar breccia as that encountered 
on the line of section. There are extensive beds of shale or clay slate 
towards the base of these mountains. 
The breccia of this formation is of a redish color. The width of the 
formation along the line of section is about 18 kilometers and here 
nearly horizontal. 1 know the formation extends from a point above 
Santo Tomás on the Balsas river to the westward beyond Coyuca and 
forms prominent mountains on both sides of the Balsas River. 
The Nanchi and Aguila Mountains are the sonthern extension or 
arm of a broad synelinal basin which extends far to the northward. 
This deposit of breccia along the Balsas river, was originally depo- 
sited in an enormous basin of erosion and ata later date eroded, form- 
ing the Balsas river valley from Santo Tomás to Coyuca and beyond. 
I have not attempted to indicate the structure of the Necoxtla slates 
further than indicate a conformability between them and the Maltrata 
limestones beyond Aguacate, where the slates appear to pass through 
gradual transitions into the limestones. 
The slates alternate with thinly bedded sandstones in places and no- 
tably near Tonelapa del Rio. 
I have not observed any fossils in the Necoxtla slates nor in the 
thinly bedded Maltrata limestones along this line of section. 
The Escamela limestones have imperfect fossil markings through the 
central supper portion of the mass. 
It has been impossible for meto make any measurements relative to 
the thickness of the formations as yet, except an approximation in the 
case of the Escamela limestones. 
At kilometer 14 of the section Eocene conglomerate ig met with and 
