180 Geoloyy of Nurtlieaatcrn West India hUiiich. 



tance, is the island of St. Croix, also with an east and west di- 

 rection. 



The rocks composing the Virgin Ishmd range are of very dif- 

 ferent kinds, massive eruptives, without stratification, enormous 

 masses of clastic volcanic rocks of most variable kinds, regularly 

 stratified metamoriDhic slates, siliceous limestones, metamorphic 

 limestone, etc., very often penetrated by black trappean dikes, 

 in the most astonishing manner resembling the trap-rocks 

 which abound in the old rocks of Scandinavia. 



The Volcanic Rocks are principally tlie following: 



1. Diorite, closely resembling syenite, is of great extent in 

 the island of Vieqne ; it occurs in a small key (Buck's Island) 

 south of St. Thomas, and farther in the southern peninsula of 

 Virgin Gorda, Avhence it may be traced in smaller patches 

 around the shores of Sir Francis Drake's Channel to the north- 

 ern point of St. John. This massive, granite-like rock consists 

 of hornblende and soda-lime feldspar, with very little mica. I 

 could not find quartz in it. It is easily altered, and shows then 

 its interior globular or concretionary structure. The small 

 island south of Virgin Gorda, called Broken Jerusalem, consists 

 entirely of large boulders of hard diorite, left after the softer 

 mass between has been carried away by decomposition, and is a 

 beautiful illustration of the globular structure of the rock. On 

 the small keys south of Drake's Channel, the massive structure 

 graduates into one more or less stratified, so that in some places 

 distinct remains of strata are visible. In other places the main 

 mass sends forth branching veins into the surrounding rocks, 

 proving that it once was in a molten state. 



The facility with which the rocks disintegrate and decompose, 

 makes it very probable that a mass of the diorite once filled the 

 whole sj)ace now occupied by Sir Francis Drake's Channel, but 

 has been carried away by alteration or denudation. 



2. Felsite. — This rock, which also could be classed as eurite, 

 or in some spots as quartz-porphyry, forms the southern part 

 of St. Thomas, St. John, and Peter's Island. It is visible also 

 on the northern part of Virgin Gorda. The color is generally 

 light, whitish, reddish, sometimes by alteration blood-red. It is 



