CURRENT PRE-CAMBRIAN LITERATURE {07 
including part of Pueblo county and the southeast corner of Fre- 
mont county, Colorado. Archean rocks occupy two small tractsin the 
southwestern part of the quadrangle. The more abundant kinds of 
Archean rocks are mica-schist, mica-gneiss, and granite. ‘The schists 
and gneisses strike north to northwest, and are nearly vertical. Their 
origin is not known. The granite is intrusive in the schists and 
gneisses. 
The Archean rocks are overlain unconformably by Paleozoic and 
Mesozoic sediments. 
Lakes’ sketches the geology of the Gunnison gold belt in Gunni- 
son county, Col., from the Cebolla River on the west to the head of 
Taylor Park and the Sawatch range on the east. The northern part of 
area is included in the granitic system of the Sawatch range. The 
southern part is occupied by schists and gneisses, underlain by coarse 
massive granite. The schists and gneisses are of pre-Cambrian age, 
but whether Algonkian or older has not been determined. The con- 
tact of the schists and gneisses with the underlying granite is an erup- 
tive one, the granite containing fragments of the schist, giving the 
impression that the schists had been floated up on an underlying 
molten or semi-molten sea of granite. Cutting the schists are occa- 
sional dikes of diabase and possibly basalt and andesite, and resting on 
the eroded edges of the schists are various later overflows of andesitic 
breccia, rhyolite, trachyte, and basalt. 
Aguilera? gives a synopsis of the geology of Mexico. The most 
ancient, or Azoic, rocks are granites, gneisses, and schists, presenting 
many variations. They extend lengthwise along the Pacific coast, 
forming a narrow band, interrupted in places, and sending rami- 
fications toward the central part of the country, in some places 
almost to the eastern coast. ‘They occupy the southern part of the 
state of Puebla, a part of the Sierra Madre Mountains in Chiapas, 
and extensive portions of Oaxaca and Guerrero; they are found also 
in Zacatecas, around Fresnilo; in Guanajuato, in the vicinity of the 
capital; in Sinaloa, around the crests of the Sierra Madre ; in Sonora, 
in its northwestern and western parts; in lower California, where 
t Sketch of a portion of the Gunnison gold belt, including the Vulcan and Mam- 
moth Chimney mines, by ARTHUR Lakes: Trans. Am. Inst. Min. Engineers, Vol. 
XXVI, 1897, pp. 440-448. 
2 Sinopsis de Geologia Mexicana, by Jos—E C. AGUILERA: Bol. del Inst. Geol. de 
México, Nums. 4, 5, & 6, 1897, Part II, pp. 189-250. With geol. maps. 
