ON THE GEOLOGY OF THE NORTH- EASTEHN WEST-INDIA ISLANDS. 37 



;i conimon rnolten igneous mäss in the interiör of the earth. The great variety in the 

 chemical composition of the volcanic rocks, even in those from the same volcano, does not 

 well harmonize with the hypothesis of an igneous molten mäss in the interiör of the earth. 

 The heat produced by metamorphosis of mountain masses of many miles in extent may 

 also be immense if it cannot escape, and may under favourable circumstances, give rise to 

 volcanic phenomena. 



The diorite of the Virgin-Islands has generally a very fine concentric-globular struc- 

 ture. It encloses in a more easily decomposed mäss rounded concretions of concentric 

 layers. These concretions are sometimes of an enormous size, being often as large as small 

 houses, and the altered surface of the diorite is mostly covered by numerous blocks ha- 

 ving the appearance of erratic blocks, which they are not by any means. 



8. Diabase is a solid greenish-black fine-grained rock, in which one can scarcely 

 distinguish the constituting minerals. In some varieties crystals of augite are scattered 

 through the mäss and by the chemical analysis one may take it as most probable that 

 the feldspathic mineral is iabradore, or perhaps also oligoklase. The black colour seems 

 to be due to a chlorite-like mineral produced by alteration of the augite. 



The diabase is very coinmon in the formation of the Virgin-Islands, where, in the 

 form of dikes, it penetrates bluebeache, diorite, felsite and sometimes, as in Whisling Cay 

 near S:t John, stratified rocks. Generally the dikes with diabase are very narrow but 

 larger rnasses also occur, as in the Hans Lollik islets north of S:t Thomas. 



The colour of the diabase is mostly dark-green or black and it is commonly fine- 

 grained, but in Mariebluff in S:t John it has an amygdaloidal structnre. Augite often 

 occurs in distinct crystals in the mäss, as in Buck's- Island, Coki Point and Hans Lollik, (S:t 

 Thomas), giving rise to augitic porphyry. As an accessorial mineral in the diabase I have 

 found epidote. Sometimes the diabase encloses fragments of the surrounding rock, and 

 the dikes are generally most fine-grained near the surrounding rock and more crystaline 

 in the middle, facts proving the eruptive nature of the rock. 



The chemical composition of the diabase of the West-Indies may be seen from the 

 following analyses. 



I. Buck ' s-Island (S:t Thomas); black with crystals of augite. Sp. Gr. 2,854. Ana- 

 lysis by P. T Cleve. 



II. Coki-Point (S:t Thomas); black with crystals of augite. Sp. Gr. 2,902. Analysis 

 by E. Eklund. 



III. Whisling Cay (S:t John); grayish-black, fine-grained. Sp. Gr. 2,895. Analysis 

 by P. T. Cleve. 



IV. Coral-Bay (S:t John); distinctly crystaline. Sp. Gr. 2,960. Analysis by P. T. 

 Cleve. 



V. Red Point (S:t Thomas); dike in felsite. Sp. Gr. 2,905. Analysis by P. T. Cleve. 



I. II. III. IV. V. 



Si 48,12 48,42 54,07 49,84 52,55 



Al 18,60 17,49 16,30 18,32 14,20 



Fe 4,00 6,79 5,75 4,16 9,43 



