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 the 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. 1, 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. 



Resume 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 in 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 Pre-Eocene age occurring on both the 



