HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 205 
2. They are all ancient rocks, and nowhere present original surfaces 
of extrusion, or evidences of volcanic vents. 
A single glance at these islands conveys two important suggestions: 
1. Their topographic forms and geologic structure are essentially those 
of the adjacent land, of which apparently they were once a part, and 
have been severed from it by subsidence or marine erosion. 2. Con- 
versely, in looking at them one is impressed with the fact that a few feet 
of submergence would convert the entire mainland of pointed hills and 
drainage valleys into exactly similar islands. They undoubtedly repre- 
sent the remnant of what was no doubt once a continuous stretch of land 
over the area now occupied by tho gulf. 
A comparison of the illustrations of this partially drowned topography 
of Panama Bay with that of the Puerto Bello coast of the mainland on 
the Caribbean side (see Fig. l, page 156) shows that it would require 
but little subsidence and erosion to submerge the valleys and to convert 
the Puerto Bello summits into islands like those of Panama Bay, 
RÉSUMÉ AND CONCLUSIONS CONCERNING THE ISTHMIAN SECTION. 
Having described the details of the general section across the Isthmus 
and the Gulf of Panama, let us examine it in order to ascertain what 
light it throws upon the Isthmian and continental history. 
For convenience the section has been arbitrarily divided in the sev- 
eral subsections under which its detailed geology has been discussed, 
In all there are seven conspicuous structural units to be noted: 1. The 
fringing coral reefs; 2. The coastal swamps of both coasts, which are 
elevated plains of sedimentation; 3. The Monkey Hill and Panama 
benches, which are elevated base levelled plains of erosion; 4. The 
folded and disturbed Tertiaries, which belong to a series of Post-Tertiary 
orogenic foldings along the Caribbean side of a more ancient nucleal 
region; 5. The numerous protrusions of basic igneous rocks which ex- 
tend back into undetermined antiquity ; 6. The sedimentary rhyolitic 
and andesitic tuffs of Barbacoas, San Pablo, Panama, and Miraflores, 
herein called the Panama formation, which in age precede the basic 
igneous formations; 7, The granitic rocks, which, as indicated by the 
detritus brought down by the Chagres, must occur Zn situ to the east of 
our section. 
Classification of the Sedimentary Rocks. — The sedimentary rocks of 
the section may again be classified by formation into three categories, 
viz: (1) Those supposedly of Pro-Eocene age occurring on both the 
