708 C. K. LEITH 
they constitute the central Cordilleran axis of the peninsula; in Vera 
Cruz, in its western region, limited by Puebla, in the canton of Zon- 
gclica. 
In the southern part of Puebla, and in Guerrero and Oaxaca, where 
the greater part of the exposures occur, the sequence is as follows, from 
the base up: (a) Porphyritic gneiss, similar to augen-gneiss, at the 
base losing its lamination and passing into a kind of granite. (4) 
Phyllite-gneiss, resting upon, and grading below into preceding beds. 
(c) Very abundant mica-schist, in some places garnetiferous, and in 
perfect conformity with the phyllite-gneisses. (d) Phyllites, very 
argillaceous in the upper part, and showing gradual diminution in the 
proportion of clay toward the base. In accordance with this change 
of composition, the structure varies from perfectly schistose to lam- 
inated, and finally to stratiform. 
After the deposition of the argillaceous phyllites, and before the 
termination of the Paleozoic, there have occurred numerous eruptions, 
in order of age as follows: Granite-gneiss, granite, granulite, horn- 
blende granite, pegmatite, greisen, and diorite. 
Comment.— The ancient rocks are mapped as Azoic, and in the text 
are described as Archean and Primitive, so that these three terms are 
used in the same sense, to cover all rocks below the Paleozoic. ‘The 
pre-Paleozoic rocks are both sedimentary and igneous, and not improb- 
ably may represent the Algonkian as well as the Basement Complex. 
Cokes Erin. 
MaopiIson, WIs. 
