STRATIGRAPHICAL GEOLOGY 
417 
STRATIGRAPHICAL GEOLOGY 
Precordillera of San Juan and Mendoza, Argentina, In the 
provinces of San Juan and Mendoza the Precordillera, a name 
introduced by Suess, is composed largely of Paleozoic sediments. 
The tract lies in a north and south line approximately east of 
Aconcagua, and intercalated between the principal Cordillera of 
the Andes on the west where marine sediments and eruptive rocks 
of the Mesozoic predominate, and the region of the Sierras Pam- 
peanas on the east where pre-Cambrian crystalline rocks abound. 
The succession of sediments of the Precordillera include those 
strata which characterize the destroyed continent of ‘‘Brasilia,” 
namely, marine deposits of the early Paleozoic succession and the 
terrestrial beds of the Gondwana group. All of the strata, save 
those of the upper portion of the Gondwana section have been 
strongly folded and elevated — supposedly by the orogenic move¬ 
ments of Tertic time. This Paleozoic structure is, according to 
Keidel,^ identical with that of the southern range of the province 
of Buenos Aires, and the mountains on the shores of South Africa, 
since in all three regions there exist glacial accumulations at the 
base of the Gondwana strata as a common characteristic. These 
accumulations have been discovered throughout almost the whole 
extent of the region in the San Juan and Mendoza provinces. 
The Precordillera occupies a tract 500 km. long, and 60 km. wide. 
It lies principally between the Rio Mendoza (S.) and the Rio Jachal 
(N.), and is cut almost in the middle by the Rio Juan. It is clear¬ 
ly delimited on the east. On the north it passes into the mountains 
of the Desert of Atacama and the Sierra de Famatina. On the 
south'it is joined with the bulk of the Cerro del Plata; and on the 
west it passes by isolated outcrops to the principal Andean ranges. 
It is unnecessary to discuss here the early observations of Bo- 
1 Observaciones geologicas en la precordillera de San Juan y Mendoza: An. Min- 
isterio agricultura Argentina. Seccion geologia Buenos Aires, 102 pp., 1921. 
