REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 23 



organic remains can be detected in the specimen, all the calcite sections showing that 

 this mineral has a stalactitic origin. The rock is almost entirely composed of sharply 

 defined crystalline grains often giving triangular sections ; by their juxtaposition 

 they present a serrated appearance. The centre of these sections is generally of a 

 brownish yellow colour, surrounded by a clearer zone ; in form they can be derived 

 from an acute rhombohedron or from a scalenohedron. This incrustation is thus 

 essentially formed of very small acicular crystals of calcite closely packed against each 

 other, the interspaces having been filled later by a calcareous deposit to which the 

 rock owes its compact and shining appearance. 



III.— ROCKS OF ST. THOMAS (WEST INDIES). 



The specimens we have examined are fragments, concerning the original situation of 

 which there is no information, some of them being rolled pebbles. It is consequently 

 necessary, in the absence of stratigraphical details, to confine ourselves to a description 

 of lithological and mineralogical features. 



One of the rock specimens has a porphyritic structure with large crystals of hornblende 

 (three to four millimetres in diameter), imbedded in an essentially felspathic ground- 

 mass. Examination under the microscope shows it to be a much altered cpiartziferous 

 diorite. The fine-grained ground-mass presents rather distinct crystals of hornblende 

 and quartz, patches of little prisms and grains of epidote and titaniferous iron, and 

 aggregations of decomposition products. Hornblende is the best developed and least 

 altered of the minerals of the first generation. The maximum angle of extinction 

 was found to be about 19°, and the characteristic cleavages are sharply marked. 

 The pleochroism is shown in the following manner : for a pale yellow ; for /8 yellowish 

 brown; and for 7 pale green; the absorption being /3> 7> a. These sections often 

 show a zonary structure, the special colours of each layer being sometimes sharply 

 defined. They are frequently twinned polysynthetically according to the law : plane 

 of twinning 00 Poo in sections parallel to 00 £ 00, in which symmetrical extinctions 

 of 19° are obtained on both sides of the lamellae. Although the hornblende is 

 relatively little altered, it is seen to be traversed by fissures which have become 

 filled by secondary quartz, probably derived from the associated minerals. Quartz 

 takes otherwise a very important place in the composition of this rock. The sections 

 show, instead of the common irregular fractures of this mineral, a series of fissures 

 which follow the cleavages of the rhombohedron. These quartz grains touch along 

 straight fines, which gives them a strong resemblance to Carlsbad twins such as 



