42 ANNALS NEW YORK ACADEMY OF SCIENCES 



to a comparatively monotonous surface for the most part at least near to 

 the sea level. The time interval need not have been of very great geo- 

 logic value, but it represents the time between the last violent outbursts 

 of volcanic eruptive activity, occurring near the close of the Cretaceous, 

 and the beginning of simple sedimentation and limestone reef develop- 

 ment and other organic accumulations in the early Tertiary. This un- 

 conformity is very pronounced along the northern margin of the island 

 ■wherever the two principal series of formations are well developed. This 

 is not easily seen on the south side. Init the relative complexities of atti- 

 tude of the two series, together with their position, emphasizes the same 

 fundamental relation. This break in sedimentation is not anywhere 

 marked by a development of a basal conglomerate. In some places the 

 new series is inaugurated by the development of shales, part of which are 

 lignitic, indicating land conditions, but in other places such beds are 

 entirely wanting and the upturned eroded members of the older series 

 are followed abruptly l)y limestones of the reef type. The tirst type of 

 succession is illustrated in the vicinity of San Sebastian and Lares and 

 the latter type of abrupt limestone succession by the conditions seen on 

 the Arecibo Eiver. It is entirely likely that the time value of this break 

 is not evervwhere the same. Probablv the districts in which shale beds 

 are developed saw the beginning of sedimentary deposition at an earlier 

 period than those in which the shales are entirely lacking. It will be pos- 

 sible to work out these historical and structural differences with further 

 study of the content and distribution. 



A very extensive development of conglomerates in the region immedi- 

 ately west of Aibonito and smaller occurrences at several other points, 

 especially on the Comerio road south of Bayamon and also near the 

 military road in the vicinity of La Muda, have a suggestion of the possi- 

 bility of a rather important structural break, but there is thus far no 

 conclusive evidence of the presence of any real unconformity. 



Veins 



Quartz veins are not prominently developed. There are occasional 

 quartz stringers and in a few places they are abundant enough to make 

 a sort of net work, but in no instance was a large persistent fissure vein 

 seen. In some cases the stringers of quartz carry values in gold, and in 

 all probability they are the source of the placer gold known to exist in 

 certain districts. So far as observations have been made, there is no par- 

 ticular system represented in the vein occurrences. Questions connected 

 with this subject together with mineralization and possible value as 

 mineral resources should ])e made a special study. 



