ORIGIN OF THE ANDES 
19 
between Los Andes and the pre-Cordillera of Argentina, the latter 
believed to be an area of old Paleozoic and pre-Cambrian rocks, 
which were folded in pre-Siluric time, according to H. Keidel.® 
West of the Andes, in Chile, there extends the great longitudinal 
valley of Chile which has the appearance of a great graben, and 
which is submerged south of Puerto Montt, in latitude 42°. The 
only other Chilean mountains are the relatively low Cordillera de 
la Costa, partially submerged south of latitude 42°. They are 
reported to consist of ancient crystalline rocks, but contain 
metamorphosed Mesozoic sediments near Concepcion, which 
throws doubt on their great age. 
The questions of pre-Cambrian history and the age of the 
crystalline rocks are seen to be one of great indefiniteness, when 
the Pliocene age of the granite batholiths and associated por- 
phyritic rocks of the Eastern Range, and the Cenozoic age of the 
intrusive rocks of the Western Range, both of which are establish¬ 
ed facts®, are considered. 
Some students have considered that the Andean geosyncline was 
compressed between the Brazilian and Argentine massifs on the 
East, and a Pacific massif on the west of which the sole existing 
remnant is that furnished by the crystalline rocks of the low coast 
ranges of Colombia, Peru and Chile. There are many reasons 
for thinking this to be probable; but it should be recalled that 
some of the crystallines in the Bio Bio valley of southern Chile are 
either metamorphosed Mesozoic marine sediments or are so in¬ 
timately associated with the latter that doubt is thrown on the 
age of the whole. 
That there was a Pacific land-mass to the west, which persisted, 
to some extent at least, as late as Miocene time, is supported by 
the following considerations: 
(1.) There is full analogy from other regions, e. g., Appalachia. 
(2.) A land area over the site of the Andes is indicated by 
the absence in most of the region of pre-Silurian deposits. 
Late Ordovicic (Arenig) graptolites were collected by Evans 
5 Uber das Alter, die Verbreitung, und die gegenseitigen Beziehungen der ver- 
schiedenen tektonischen Strukturen in den argentinischen Gebirgen: Comp, rendu, 
XII, Cong. Geol. Int., pp. 671-687, 1914. 
6 Proc. U. S. Natl. Mus., Vol. LV, pp. 279-294, 1919; also, J. T. Singewald; 
Economic Geology, Vol. XVI, pp. 60-69, 1921. 
