GEOLOGY AND MINERALOGY. 
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thence northwest, crossing the Almoloyo and Malatengo rivers. 
In this latter situation it is of a less metaraorphic character than 
near Tarifa, or on the southern flanks of the Masahua hills : 
where the Malatengo crosses, it is of a dark-blue color and unal- 
tered. It is intersected by thin thready seams of calc spar ; is 
apparently a mud limestone, not fossilliferous, and of the Silu- 
rian series. It is a valuable building material. 
North of La Chi vela, and extending from the Rio de Chicapa 
in a northwest direction to the hill of Guie-xila, Silurian sand- 
stones and slates, quartz and talcose rocks, form the surface 
rock, intersected with layers of white quartz. These constitute 
almost the chief rock of the summit-level. La Chivela is placed 
upon quartzose sandstone, which has a slight slope to the north- 
east. 
The crescentic margin of the 'limestone described previously 
is overlaid by a sandstone rock which slopes northerly and east- 
erly, upon the surface of which the rivers roll ; from Guichicovi, 
and for several miles to the east, as far as the Coatzacoalcos this 
sandstone becomes more ferruginous, and passes into jasper and 
claystone. Porphyritic alterations of the rocks here are exten- 
sive. On the banks of this river, south of where it receives the 
Malatengo, primary limestone crops out, as also in the stream. 
The above sandstone is of a purple color, slaty and semi-crys- 
talline. It is a durable stone, and inclined to split into flags. 
It forms the surface rock of the northerly course of the Malaten- 
go, and the country thence crossing the Sarabia and Jumuapa. 
Along the line of survey it has a gentle dip to the north- 
east of a few degrees. Between the Sarabia and Jumuapa 
rivers the ground is elevated by porphyritic and greenstone 
rocks ; the sandstone, however, sutlers little displacement. 
South of the Jaltepec River, along the line of survey, the 
compact limestone again crops out from under the sandstone, 
and is accompanied by thin veins_ of black quartz or horn- 
stone. The upper bed of this sandstone is a conglomerate, 
and occupies a considerable surface extent between the Jaltepec 
and Jumuapa, south of the limestone. Its direction is described 
further on. 
Commencing at a point where the Rio Pachine joins the Ma- 
