468 OLIVER C. FARRINGTON 
of the orographic system known as the Cordillera of the Andes 
It is in the part of Mexico known by the name of the Western 
Sierra Madre that.we may expect to find the principal types of 
ancient eruptive rocks, associated as a rule with crystalline 
schists and some of the earliest sedimentary rocks. The Sierra 
Madre extends along the Pacific coast in a general southeast- 
northwest direction. The western slopes, generally descending 
rapidly to the coast, present a notable contrast to those of the 
east, where numerous spurs or secondary sierras serve to support 
the extensive plateau of the Mesa Central. It was along the 
eastern slopes that the eruptive movements which in epochs 
later than the Cretaceous added to the relief of the Sierra 
Madre, chiefly occurred. Here may be seen the whole series of 
modern eruptive rocks, from the granites with which the erup- 
tions clearly began, to the basalts of Quaternary time. 
Considering first, then, the pre-Cretaceous rocks, we find 
them consisting chiefly of granites. These probably make up a 
large part of the mountains along the western coast of the Penin- 
sula of Lower California, but of their exact distribution we 
know as yet little. In the region of Hermosillo, Sonora, in the 
district of Moctezuma, granite, crossed by dikes of pegmatite, 
occupies great tracts of country. Inthe district of Altar sye- 
nites and diorites replace the granite. In the region south of 
the state of Puebla in the districts of Chiautla and Matamoros 
micaceous or amphibolic granites are found passing over to 
gneiss or green schists. The granites are interrupted frequently 
by modern eruptive rocks, chiefly rhyolites and andesites, or 
even by stratified rocks, generally Cretaceous. In the state of 
Jalisco in the canton of Mascota and along the slopes of the 
Sierra toward the Pacific, the group of mountains of Desmoron- 
ado is formed of granites associated with quartzites and other 
metamorphic schists. In the central and western parts of the 
state of Oaxaca may be seen an extensive formation of granites 
and diorites, covered sometimes by breccias and modern quartz- 
ose conglomerates. These ancient masses, chiefly granites, may 
be followed, although interrupted by modern eruptive and sedi- 
