664 REVIEWS 
the age of which is not definitely known ; (6) the sedimentary rhyolitic 
and andesitic tuffs, the Panama formation, older than the basic igneous 
formations; and (7) the granitic rocks, the oldest of the region. The 
sedimentary rocks of the section fall into three categories: (1) those 
of supposedly pre-Eocene age, occurring on both sides of the Isthmus ; 
(2) the fossiliferous Tertiary beds of the Caribbean side, and (3) the 
Pleistocene beds deposited synchronously on both sides. 
The rocks of the first of these three classes are so distorted and 
concealed by later igneous protrusions and deposits that little was 
learned of their relations. They are composed almost entirely of 
rhyolitic and andesitic material. The beds of the second class are 
referable, on the basis of their fossils, to the Eocene and Oligocene, 
corresponding approximately with the Tejon, Claiborne and Vicks- 
burg formations of the United States. They are composed mainly of 
muddy sediments with more or less sand, glauconite and lignite. 
Limestone occurs at two horizons. It is to be specially noted that the 
Tertiary deposits of the Isthmian section end with the Oligocene 
(early Miocene), and there is nothing to indicate that sedimentation 
was in progress in the Isthmian region during the later Miocene and 
Pliocene epochs. ‘The inference is that “the Isthmian land was of 
much larger area during these later epochs than in Eocene time or at 
present.” The thickness of the Tertiary system is thought to be as 
much as 1000 feet, and possibly much more. 
The igneous rocks of the Isthmian section consist of (1) granite in 
ranges having an east-west trend; (2) rhyolitic tuffs and pumice, 
igneous in origin, but sedimentary in arrangement; and (3) basic 
igneous rocks, occurring as intrusives, eruptives, tuffs, etc. The basic 
rocks are younger than the others, but are thought to have been in 
existence during the deposition of the later Eocene sediments. Other 
considerations prevent the reference of these rocks to an earlier period, 
and lead to the conclusion “that the most marked volcanic episode of 
the Isthmian region took place during the later Eocene epoch.” It is 
thought that there are also late Tertiary syenitic intrusives, which 
“now form the core of great mountainous protuberances.” 
Throughout the Isthmian region, the surface red clays conceal all 
the formations. They are believed to be largely the residuum of rock 
decay. The extent of this decay is said to be “ enormous, extending 
often to a depth of over 100, and seldom less than fifty feet.” 
In the Costa Rican sections the principal geological features may 
