salt-deposits.--Metalliferous veins.--Summary on the porphyritic
conglomerate and gypseous formations.--Great subsidence with partial
elevations during the cretaceo-oolitic period.--On the elevation and
structure of the Cordillera.--Recapitulation on the tertiary series.--
Relation between movements of subsidence and volcanic action.--Pampean
formation.--Recent elevatory movements.--Long-continued volcanic action in
the Cordillera.--Conclusion.



INDEX.




GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA

BY

CHARLES DARWIN.




CRITICAL INTRODUCTION.

Of the remarkable "trilogy" constituted by Darwin's writings which deal
with the geology of the "Beagle," the member which has perhaps attracted
least attention, up to the present time is that which treats of the geology
of South America. The actual writing of this book appears to have occupied