ON THE GEOLOGY OF THE NORTH-EASTERN WEST-INDIA ISLANDS. 35 



gradual passages from felsite to a kind of fine-grained talc or mica-slate are to be found. 

 In some places the félsite graduates into clay-slate as on the north side of S:t Thomas, 

 near Maho Bay. 



6. Syenite-Porphyry occurs in the eocene formation of S:t Bartholomew and probably 

 also in S:t Martin, where I found loose boulders of this rock. 



It appears to be a mixture of orthoklase, oligoklase and very little hornblend. It 

 does not contain quartz in any quantity. The inass is gray or dirty greenish ; compact 

 and contains many small white crystals of feldspar or black shining needles of hornblend. 

 The rock is evidently eruptive, it has a parallelopipedic or more commonly globular- 

 concentric structure. It is very härd and is sometimes employed as building-stone in the 

 town of S:t Bartholomew. 



Its Sp. Gr. is 2,61, and one analysis by P. T. Cleve has given: 



Si 66,28 Äl 16,23 Fe 2.n Fe 1,62 Ca 4,03 Mg 1,03 Na 3,36 K 1,60 H 2,65. Sum. 99,51. 



7. Diorite. A crystaline granular mixture of oligoklase, hornblend and some mica. 

 The rock occurs to a great extent in the cretaceous formation of the Virgin-Islands, and 

 also in S:t Martin, which latter is probably of eocene formation. In Crab Island the dio- 

 rite is the ruling rock; in S:te Croix also diorite is noticed near South-Gate and in the 

 small rock called Green Cay, in which latter spöt the rock is very coarse and contains 

 some quartz. The little BuckTsland near S:t Thomas consists of diorite, penetrated by 

 numerous trap-dikes. In the islands bordering the Francis Drake's Cbannel, and also in. 

 Virgin-Gorda and S:t John, the diorite occurs, and it is very probable, that the channel has 

 been excavated in this rock of comparatively easy decomposition, which has originally 

 formed an enormous large mäss here situated in the synclinal axis of dipping metamor- 

 phic and stratified rocks. 



The diorite of the Virgin-Islands has a great resemblance to syenite, being of a 

 whitish colour from feldspar preponderating. 



The feldspar is a variety of oligoklase containing much lime (Hafnefjordit). The 

 analysis of that mineral has been given previously. 



Hornblend occurs in a smaller quantity. It has a greenish black colour. 



Greenish or bronze-coloured mica is to be regarded as an almost constant corjpo- 

 nent and the rock may consequently be classed as Mica-diorite. 



The diorite contains a little magnetic iron, however only a very small percentage. 

 Among accessorial minerals found in the diorite I may mention epidote in crystaline masses 

 or uncrystalized compact concretions (Browns Bay, S:t John); titanite in small almost micro- 

 scopical honey-coloured crystals; garnet of brownish colour and small quantities of copper- 

 minerals. In fissures in the diorite desmine, carbonate of iron and white quartz are some- 

 times found. The rock is often penetrated by numerous dikes with diabase or granular 

 granite. 



The following analysis will show the chemical composition of the diorite: 

 1. Diorite from Marys Point in S:t John, Sp. Gr. 2,801. contains only traces of 

 magnetic iron. The analysis of the feldspar is given above. Analysis by P. T. Ceeve. 



