MUSEUM OF COMPARATIVE ZOOLOGY. 255 



irreo-ularities having been levelled by solution. The formation averages 

 about thirty feet in thickness, and usually extends inland only a short 

 distance, often only a few yards, as on the northwest end of Moro Point, 

 or not over an eighth of a mile, as at Baracoa, and is especially well 

 exposed along the narrow points of the numerous small harbors, as shown 

 on Plate II. 



The soboruco is a topographic as well as a stratigraphic feature, for its 

 surface is a bench gently sloping to the sea ; it has neither been covered 

 by later deposits nor greatly denuded. It usually forms a cliff at the 

 surf line, about fifteen feet in height, against which the surf beats with 

 great force, wearing deep indentations. The spray breaks over the 

 summit, with the aid of the sun producing the surface induration which 

 is visible wherever rain or other moisture falls upon the hot limestones, 

 or w'earing the surface into cavernous Karrenfelder. This solution and 

 induration at Baracoa, for instance, has converted the limestones in 

 spots into a coarse saccharoidal marble, and has aided in the segregation 

 of small lumps of iron ore direct from the coral. 



Where I was able to examine the base of the elevated reef rock, 

 mostly at the mouths of rivers, it seems to have been deposited rather 

 abruptly upon a semi-argillaceous terrane of silt, and occasionally very 

 fine pebbles, which have been brought down and deposited by the rivers. 

 (Plate II.) I did not find it growing upon the larger gravel which is 

 deposited immediately at the river's mouth, as is seen off the Yumuri of 

 Baracoa, where the river empties into the sea, and not into a bay. Further- 

 more, the present submerged fringing reefs do not grow immediately 

 where the rivers send their fresh waters into the sea, but are interrupted 

 there by a barren area simulating a submarine channel, as is shown in 

 the accompanying illustrations of the harbors. This fact has an impor- 

 tant bearing upon the origin of the present circular harbors, and upon 

 tlie theories of alleged subsidence, both of which subjects are more fully 

 discussed in later pages. 



Is is impossible to describe all the localities at which the soboruco was 

 observed.^ Sometimes, as along the Havana coast, it occupies a narrow 

 coastal strip extending from the])oint of one harbor to another. Again, 

 as on Moro peninsula, opposite Havana, it occurs only as a small patch 

 in a slight indentation in the old headland composed of folded Miocene 

 rocks. (Plate I. Fig. 2.) 



At Tanamo and other places on the north coast the soboruco not only 



1 See A. Agassiz, Bull. Mus. Conip. ZooL, Vol. XXVI. No. 1, Plates XLIV.- 

 XLVII. 



