106 



ON THE TOPOGKAPHY AND f^EOLOCJY 



stream the i^^igua produced but little ettect, and judging from the absence of gravel 

 in its vicinity must have been even more unimportant than one might have inferred. 

 The ISTizao brought down its quota which is distributed as far east as the Miocene 

 hills of San Cristobal. But still more marked is the immense sheet of gravel, a 

 hundred feet thick, and with boulders of half a ton furnished by the Southern Yacpii, 

 and which covers all the plain on which Azua is built, and aided by contributions fi'om 

 the Ocoa which extends eastward along the coast, ahriost to the JS^izao. East of the 

 Isabella the hills suddenly retire to the northward and the larger rivers like the 

 Ozama and Macoris run almost entirely in the plain, so that at that period only their 

 upper branches existed ; streams too unimportant to produce a marked effect in the 

 formation or modification of the sea-bottom. 



But to return to the Jaina as an exemplification of the whole. The gravels are 

 spread over the low-rolling hills and the plains of the Porto Kico and Sta. Ko^a 

 savanas, every pebble telling its own story of its origin, — the beds, except in the 

 presence of a larger proportion of sand, being a repetition of the river bars of their 

 parent stream. But as the distance from the ancient coast increases the pebbles 

 become scarcer and smaller and the finer portions more abundant, until in the Savana 

 la Venta the beds are almost entirely sand and clay with a few straggling pebbles, 

 such as might have been transported by floating roots, &c. And here a new element 

 begins to come in. In the Yenta the dejDosit begins to exhibit a distinct calcareous 

 character. This increases, and within the distance of a mile or too the sand and clay 

 strata are entirely replaced by the deeper water deposit of the coral reefs. But in 

 going eastward it will be seen that the curi-ent carried along with it the finer particles 

 of the river debris and spread it like a ribbon, parallel with the old coast line, receiving- 

 trifling contributions from the smaller streams until the supply became exhausted, and 

 then the coral deposits approached the coast until they came in contact east of Higuey. 

 It is not necessary to suppose that the limestones were deposited in very deep watei-, 

 the presence of considerable quantities of fresh water from the rivers with the accom- 

 panying mud would account fully for the absence of corals near the coast. Besides 

 the presence in two or three spots of abundance of shells of Ostrea, Lucina and of 

 Yener'ulce, and the additional fact that casts of Stromhus are nowhere rare, indicate 

 that at least a part of the rock must have been deposited in comparatively shallow 

 water, probably as an encircling reef similar to those existing around the eastern end 

 of the present island and elsewhei'c. Lyell* devotes several pages to the discussion 

 of the theory of coral reefs and atolls, and quotes the objection of Mr, Maclaren 

 that no reefs have ever been elevated above the surfacje of the sea. That gentleman 



* Piiuciples of Geology, 9tli Ed.,-p. 793, e?; seg. ■ 



