308 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, No. 14 
around Fajardo are correct, but he gives no data of assistance in 
determining their geologic age. 
Summary statement on Cretaceous rocks.—Fossiliferous Upper 
Cretaceous sediments interbedded with voleanic rocks occur in 
Saint Croix and Saint Thomas. Sediments with interbedded or 
associated voleanic rocks similar to those of Saint Croix and Saint 
Thomas are present in Saint John, Culebra, Vieqeus, and eastern 
Porto Rico, and because of similar lithology and similar deformational 
history are considered of Upper Cretaceous age. 
Early Tertiary Events 
Subsequent to the deposition of the Upper Cretaceous sediments and 
the extrusions of the associated voleanic rocks, there was intense 
deformation, which resulted in tight folding in Saint Croix, where dips 
of 80° or more are common; the dips in Saint John are about the same; 
in Saint Thomas they are 50° or more; in Culebra I noted some low 
dips, only 19°; in Vieques the dips are as much as 60°; in eastern 
Porto Rico the range is from about 13° to almost vertical. Some of 
the older igneous rocks, which in many instances have been so crushed 
that they are now chlorite or sericite schists, may be of pre-Cretaceous 
age. ‘The structure lines cannot be described at this place—only the 
general statement may be made that in places there are intersecting 
trends, one approximately east and west and another from northwest 
to southeast. The two sets of trends are very clearly recognizable in 
Saint Thomas. There were extensive intrusions of diorite, dolerite, 
and quartz-diorite, and probably also extrusions of lavas and tufts. 
Quartz-diorite is the dominant rock in Vieques. The older series of 
- rocks was subjected to subaerial erosion for so long a time that over 
considerable areas they were practically base-leveled and the next 
younger sediments were laid down on a nearly plain surface developed 
on rocks that dip as steeply as 80°, as in the southwestern part of 
Saint Croix. 
Tertiary Sediments 
Sediments of Tertiary age are present in Saint Croix, Vieques, and 
eastern Porto Rico but appear to be entirely absent on Saint John, 
Saint Thomas, and Culebra. In fact, except Anegada, there are no 
known Tertiary sediments on the axial islands of the Virgin group, 
and in Porto Rico such younger deposits are confined to the northern 
and southern flanks of the island. Therefore the axial islands of the 
Virgin Bank and the sierras of Porto Rico appear to have been above 
water since the close of Cretaceous or since very early Tertiary time. 
