BERKEY, GEOLOGICAL RECONXOISSAXCE OF PORTO RICO 15 



mation were seen, and this is true also of the southerly side coming from 

 the east end through Xaguabo and Yabucoa and Guayama, and still far- 

 ther west to some undetermined point l^etween Guayama and Santa Isa- 

 bel. There mav be, however, occurrences of this formation underlvinj; 

 the alluvial material along the coast at points considerably nearer Guay- 

 ama than the outcroj) map indicates. 



The formation as a whole is essentially a structural unit. Although 

 it is quite easy to distinguish the underlying shale member, especially 

 well developed between San Sebastian and Lares, and although there are 

 other structural changes, there is nowhere any appearance of unconform- 

 ity or marked break in the succession. Besides, the shale member is not 

 everywhere developed and, as a matter of fact, is seldom seen in tracing 

 the formational boundaries. In some places, it is definitely shown to be 

 absent, so that it seems unwarranted to represent this member as an im- 

 portant part of the formation, so far as areal distribution is concerned. 

 Judging from field observations already made, the shale and marl beds 

 are more extensively developed on the south side than elsewhere. But 

 this is based on observations in one particular area and the member can- 

 not be traced very far in either direction because of other difficulties. 

 The beds lying above the shales and representing the part referred to as 

 a more massive limestone portion are probably susceptible to considerable 

 differentiation on the basis of fossil content, and it is entirely possible 

 that a rather complete range of Tertiary horizons may be determined 

 after complete paleontologic study. In the field, however, and on the 

 basis of structural factors, there is no apparent ground for subdivision. 

 In this discussion, therefore, the whole series of beds, from the uncon- 

 formity at the base to the alluvium and San Juan formation overlying it, 

 is referred to as a single formation and, on account of its extensive devel- 

 opment in the region about Arecibo, it is suggested that a suitable name 

 would be the Arecibo formation. 



Some parts of the formation show the peculiarities and content of a 

 coral reef, and these portions have the most irregular and most massive 

 structures. Other parts show bedding structure more or less perfectly de- 

 veloped, and throughout the whole formation here and there, at irregular 

 intervals, and usually of only very limited extent, there are more shaly 

 fades. It is the opinion of the writer that this irregular distribution of 

 shaly beds is responsible for one of the peculiar topographic features de- 

 veloped in the belt underlain by the Arecibo formation. This is the 

 occurrence of almost perfectly flat soil-covered areas of no very great 

 lateral extent at different levels, above which the numerous small knobs 

 or hillocks of limestone rise, giving the peculiar haystack -like topography. 



