VEINS MOST FREQUENT IN THE OLDER ROCKS. 549 
deposited in successive, and often corresponding 
layers on each side of the vein. 
Metallic Veins are of most frequent occurrence 
in rocks of the Primary and Transition series, 
particularly in those lower portions of stratified 
rocks which are nearest to unstratified crystal- 
line rocks. They are of rare occurrence in 
Secondary formations, and still more so in Ter- 
tiary strata.* 
* M. Dufrenoy has recently shewn that the mines of Haema- 
tite and Spathic iron in the Eastern Pyrenees, which occur in 
Limestones of three ages, referrible severally to the Transition 
Series, to the Lias, and to the Chalk, are all situated in parts, 
where these Limestones are in near contact with the Granite ; 
and he considers that they have all most probably been filled by 
the sublimation of mineral matter into cavities of the limestones, 
at, or soon after the time of the Elevation of the Granite of this 
part of the Pyrenees. The period of this elevation was posterior 
to the deposit of the Chalk formation, and anterior to that of 
the Tertiary Strata. These Limestones have all become crys- 
talline where they are in contact with the Granite ; and the Iron 
is in some places mixed with Copper pyrites, and Argentiferous 
galena. (Memoire sur la Position des Mines de Fer de la Partie 
orientale des Pyrenees, 1834.) 
According to the recent observations of Mr. C. Darwin, the 
Granite of the Cordilleras of Chili (near the Uspellata Pass) 
which forms peaks of a height probably of 14,000 feet, has been 
fluid in the Tertiary period ; and Tertiary strata which have 
been rendered crystalline by its heat, and are traversed by dykes 
from the granitic mass, are now inclined at high angles, and 
form regular, and complicated anticlinal lines. These same sedi- 
mentary strata, and also lavas are there traversed by very nu- 
merous true metallic veins of iron, copper, arsenic, silver, and 
gold, and these can be traced to the underlying granite. (Loud, 
and Edin. Phil. Mag. N.S. Vol. 8, p. 158.;) 
