HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 175 



made by this elevation once extended over at least what is now Man- 

 zanilla Bay and Limon Bay, and probably out seaward some distance. 

 Hence it is equally certain that since these late sediments were thus 

 elevated into land the former seaward extension of the coast has been 

 partially destroyed and its area diminished by the destructive surf 

 erosion. 



That the Isthmian region was much narrower than now when these 

 swamps deposits were below sea level is very evident, especially when we 

 consider that similar phenomena occur on the Pacific side for at least 

 four miles inland from the present margin of Panama Bay. The waters 

 of the two oceans now forty-five miles apart, were then fifteen miles 

 nearer together. Let not those who believe that the two oceans were 

 recently united across the Isthmus interpret this former restriction of 

 the Isthmian land as demonstrative of their theory, for it can be shown 

 that when these swamp levels were submerged the two oceans were 

 still separated by a land barrier of even greater height than that of 

 to-day. 



Subsection 1. — Colon to Bujio. 



Leaving the depot at Colon the railway first passes the swampy land 

 of the lower end of the island, and then crosses the narrow strait known 

 as Fox River, which connects Limon and Manzanilla Bays. 



The Monkey Hill Base Levelled Terrace. — Beyond this swamp a low line 

 of hills is met. These are lobed and cut into many individual members, 

 but the eye recognizes readily that their summits, less than a hundred 

 feet in height, are the remnants of a once continuous bench or level. 



Figure 2. Monkey Hills, near Colon. 



The physiography of the Monkey Hills is of great interest. Their 

 uniform summit level, traceable a few miles eastward, parallel to the coast, 

 cleai'ly represents an ancient base levelled surface, probably what was the 

 low coast border when the Mindi swamps were below the waters of the sea. 

 Although this old level has been subjected to later erosion, so much so 

 that it is now dissected into numerous mammillary hills, there can be 

 no misinterpretation of the fact that their summits were once a con- 

 tinuous plain. We may own that this old plain once included Lion and 

 Tiger Hills to the southward, now separated from the Monkey Hills 

 proper by a vast area of swamp. I am inclined to believe, however, that 



