ON THE GEOLOGY OF THE NORTH-EASTERN WEST-INDIA ISLANDS. 39 



are imbedded numerous white crystals of glassy feldspar (easily melted by the blow-pipes) 

 some needles of hornblend and also very few small greenish crystals of augite. 



The Sp. Gr. is 2,71. 



The rock of Saba has been analysed by Dr. Th. Nordström. 



Si 60,80 Äl 16,34 Fe 0,68 Fe 5,14 Ca 6,92 Mg 1,47 Na 6,71, K 1,12 H 0,37. Sum. 99,55. 



The small quantity of potash as well as the presence of much lime and sodium 

 prove that the rock is chiefly composed of oligoklase (andesine?) or microtinite, and as the 

 low quantity of silica indicates the absence of quartz, this trachyte may be classed as mi- 

 crotinite without quartz. 



Near the sulphur-mines of Saba the trachyte is cbanged to a kind of alum-stone. 

 Such an altered trachyte has been analysed by Mr. Th. Fiebelkorn who has found it to 

 contain : 



Si 34,io S 32,oo Äl 17,82; AI 2 Cl, 5,47 K Cl 0,io H 12,oo. Sum. 101,49. 



Thus by action of vapours containing sulphuretted hydrogen the rock has been de- 

 prived of its iron, lime, sodium, and a part of the silica, the former having been trans- 

 formed to soluble sulphates and swept away. 



12. Mica Schists are found in subordinate strata in the small Virgin-Islands as Salt 

 Id, Peter's Id and in the northern part of S:t John. The mica always occurs in small 

 dark scales. Schists of grayish or reddish colour and composed of very small scales of 

 talc or mica occur near Caroline Estate in S:t John. It graduates into felsite, from which 

 rock it seems to have originated by metamorphosis. In other places the mica schists seem 

 originally to have been clay-slate as in the small islets between S:t John and S:t Thomas, 

 and in Whisling Cay near S:t John. 



13. Clay-slate. Black clay-slate, containing no fossils and in appearance resembling 

 silurian clay-slate, is in the island of S:te Croix an important rock, constituting several high 

 hill or mountains. It is also found in the northern part of S:t Thomas and S:t John. In 

 Tortola I found the rock near Coxheat. 



By metamorphosis the clay-slate is transformed in some places (as in S:t Croix, near 

 Goathill, and at a spöt near Christianstaad) to lydian stone or to a kind of härd gray 

 chert or quartzite. 



In other places may be seen gradual passages from clay-slate to mica slate, horn- 

 blend-slate and diorite. The smaller islands south of Sir. Fr. Drake's Channel, as also the 

 north of S:t Thomas and S:t John, afford a very good opportunity for the study of 

 those rocks. 



14. Chert-breccia (Hornsteins-breccia). As such I consider a yellowish-white alveo- 

 lar rock, probably originating from metamorphosis of volcanic breccias. It occurs in S:t 

 Eartholomew in great extent near Grand fond. 



15. Limestone-breccia, a rock composed of angular pieces of yellowish-white lime- 

 stone cemented by a fine-grained red matter, occurs in fissures in the miocene formation 

 of Anguilla. It contains bones of extinct mammals and shells of still living air-breathing 

 mollusca. In the composition of the rock there is also some phosphate of lime. 



