188 G eulogy of Xortlieasterii West India Idauch. 



o. Limestone. — Hard, crystalline, bluish-gray marble, occurs 

 on Congo Key and on Great Patch Island, near St. John ; also 

 on Mary's Point, on the latter island, and in some other places. 

 Sometimes the lime is so strongly impregnated with silica that 

 it forms a peculiar, still stratified vock, ^siliceous limestone, — 

 which is filled with garnet, epidote and other silicates. Near 

 Coki Point of St. Tiiomas, the limestone stratum thins out 

 in small rounded lime boulders, which occur imbedded in the 

 blu3-beach rock. Th3se rolled pieces contain fossils, sometimes 

 silicified and well preserved, which allowed me to make out with 

 certainty the geological age of the Virgin Islands. 



The Island of St. Croix consists in its northern part of clay- 

 slates, blue-beach, felsitic rocks, and some limestone, all of the 

 same character as in the Virgin Islands, dipping at very high 

 angles, often almost vertical, and running, though with many 

 deviations, from west to east. South of this rocky part of the 

 island, extends a wide level area of coral limestone and marls, 

 probably of Miocene or perhaps more recent date. 



Fossils of the Cretaceous Formation. — The fossils collected 

 near Coki Point, on St. Thomas, were abundant fragments of 

 a large Nerincea, Actceonella, Pectunculus, Astarte, Corhula, Li- 

 mopsis, Opis, and one Ammonite. All these fossils prove the 

 age to be Cretaceous, and probably corresponding to the Gosau 

 formation in the Alps. 



The Cretaceous formation of the Virgin Islands and St. Croix 

 consists, then, chiefly of volcanic rocks, often stratified and 

 associated with large eruptive masses of a light colored diorite, 

 closely resembling syenite. Their strike is ^generally east to 

 west, and their dip very strong, which proves that they have 

 been elevated and bent by a great pressure, acting from north 

 or south at a right angle to the strike of the strata. On study- 

 ing in detail the part between Tortola, St. John and St. Thomas, 

 I found that there is a synclinal fault just in the continuation 

 of Sir Francis Drake's Channel. Tortola and St. John, with its 

 continuation, St. Thomas, are only parts of the same large set 

 of strata, as will be clear by the accompanying schematic 

 section (Plate XVII). 



