10 AXXALS XEW YORK ACADEMY OF SCIENCES 



narv work of this expedition was devoted primarily to this question and 

 collections were made for comparison- throughout the island. In the 

 beginning, observations were made chiefly along the coastal margins be- 

 cause of the greater amount of ground that could be covered and because 

 of the apparent simplicity of the outermost and younger or more recent 

 formations. The chief formations with their representative rock variety 

 will Ije discussed in order from the yomiger to the older series. 



In the first place, such a reconnoissance shows that there are two great 

 series of formations separated by a marked unconformity. Both are 

 somewhat complex, but in that respect the older series is very much more 

 complex, both in range of composition and number of units involved and 

 in variety of structural relation, than the younger one. In spite of this 

 discrepancy, it is still the most convenient and useful division to make, 

 and, because of the strikingly different characteristics of the two series 

 and the great prominence of the structural break between them, there is 

 no possible chance for mistaking this fundamental feature. 



The whole lot of formational units are therefore grouped under the 

 f ollowi. ^ two heads : 



1) Younger Series. 



Including tlie Tertiary shales, reef limestones and recent deposits. 



2) Older Series. 



Including a complex group of formational units, — tuffs, ashes, shales, 

 conglomerates, limestones and a great variety of intrusives, all of which 

 are probably of pre-Tertiary age. 



There are several possible subdivisions of the younger series, but in 

 this discussion only those exhibiting enough physical constancy and char- 

 acter to be useful in field correlation are taken into account. These are 

 especially (1) the San Juan Formation, a Pleistocene sand-dune deposit, 

 and (2) the Arecibo Formation, a series of reef limestones and associated 

 shales and marls. Besides these, there are more local developments that 

 deserve special discussion, such as the San Sebastian shales, the Juana 

 Diaz marls and sandy shales, and the Ponce chalk beds ; but in a broad 

 grouping these are all phases of the larger Arecibo Formation and it will 

 take detailed paleontologic study to make the proper subdivisions. 



The older series has many formational members and their general rela- 

 tions are reasonably well understood, but a systematic subdivision is not 

 yet attempted. Correlation in this series is still more difficult than in 

 the other, because of the great variation in character laterally and the 

 influence of igneous activities that prevailed throughout its whole devel- 

 opment. Some of the most characteristic of these types will be described. 



