ON THE GEOLOGY OF THE NORTH-EASTERN WEST-1NDIA ISLANDS. 29 



not crystaline masses, accornpanied by malachite and chrysocolla. The mineral is very 

 loose and bas the sp. gr. 2,06. It is easily decomposed by hydrochloric acid. 

 Composition according to an analysis by Mr Th. Fiebelkorn. 



Si; 35,08; Cn 23,18; Fe 9,91 ; H (volatilized by 100°) 23,15; water vaporized by igni- 

 tion 8,53; Sum. 99,85. 



All the iron occurs as peroxide, but if, as is very probable, it originally has been 



in the mineral as protoxide, one may calculate from the analysis the formula R 2 Si 3 -f- 



qaq., in which R = 1 / s Fe and 2 / 3 Cn. 

 This formula requires. 



Si 36,33 Cu 21,30, Fe 9,68 H 32,69, Sum. 100,oo. 

 • Of the nine equiv. of water in the mineral six seem to be löst at 100°. 



27. Orthoklase, red crystaline feldspar occurs as an ingredient in pegmatite-granite 

 in several places in the Virgin Islands, as near Belmont on Tortola, in Beef-Island and 

 Great Thacht Island near Tortola. The glassy feldspar of the trachyte of Saba etc. seems 

 to be oligoklase. 



28. Oligoklase occurs in tolerably large crystaline pieces in pegmatite-granite in se- 

 veral places, as in Beef-Island near Tortola, in Round Rock near Virgin Gorda etc. Oli- 

 goklase is an ingredient of the granular granite and felsite of the Virgin-Islands as also 

 in the syenite-like diorite of these islands. The oligoklase occuring in the latter rock is 

 lime-oligoklase or Hafnefjordite. The Hafnefjordite from the diorite in Marys Point at S:t 

 John has a sp. gr. of 2,687, and contains according to an analysis by P. T. Cleve: 



Si, 59,45; Äl 25,52; Fe 0,70; Ca 7,88; Na 4,57 Loss by ignition 0,63 Sum. 98,74. 



The ratio of oxygen in R : R : Si : : 1 : 3,5 : 9,3 and the formula is consequently 

 2 (Na Ca) O, 3 Si 2 + 2 (Al, 3 . 3 Si 2 ). 



29. Labradorite probably occurs as an ingredient in the diabases of the Virgin- 

 Islands, and also in the Labrador Porphyry of Culebra, in which rock it forms small white 

 crystals. 



30. Anorthite is found as an ingredient in the Anorthite- Diorite of Beef-Island. The 

 mineral is whitish-gray semi-transparent, crystaline and has the sp. gr. of 2,78. 



Analysis by P. T. Cleve. 



Si 45,60; Äl 31,96; Fe 2,08; Ca 17,50; Mg 0,59; K 0,47: Na 1,10 loss by ignition 0,45. 

 Sum. 99,75. 



The ratio of oxygen in R : R : Si : : 1 : 2,7f; : 4,33. or 1:3:4 and the formula is con- 

 sequently Ca O, Si 2 -j- Al 2 3 . Si 2 . 



Anorthite from S:t Eustatius is analysed by H. S:t Claire Deville (Ann. de Chim. et 

 de Phys. [3] 40 p. 286). 



31. Epidote is very common. Fine crystals are found in Puerto Rico (Rio blanco 

 and Rio Prieto); in S:t John (Virg. Isds) at Brown's Bay in syenite, at Belmont (Tortola) 

 commonly accornpanied by garnet; on Bucks Island (S:t Thomas) uncrystalized in diabase. 

 On Bucks Island near S:t Croix smaller granules of epidote occur densely imbedded in a 



