REPORT ON THE PETROLOGY OF OCEANIC ISLAND& 19 



In the brownish vitreous mass there are numerous small microliths of augite, 

 almost colourless, or of slightly purplish tint ; these crystals are often grouped in the 

 form of a cross or star, but it was not possible to ascertain the law of this intercrossing. 

 The zeolites, as is generally the case with the rocks of this type, have been formed in 

 drusy cavities, lining them with a rather thin layer, which is almost colourless or only 

 slightly bluish between crossed nicols. 



A rock with an enamelled calcareous coating, found on the coast, must also be 

 classed with limburgite. It is black, more massive than the preceding, slightly 

 vesicular, and has the macroscopic characters of basalt. Examined with the micro- 

 scope it is seen that all the constituent elements are the same as in the rock just 

 described ; its base is, however, less developed, and all the microporphyritic crystals, 

 especially those of augite, are of a larger size. The olivine shows its cleavages in a 

 more distinct manner, and it is penetrated and corroded by the magma. The vitreous 

 mass is less homogeneous, and less transparent than in the rock last described ; in some 

 places it is filled with trichites, and irregular granules of magnetite. 



A rock specimen broken from a steep cliff near the slaughter-house of Porto Praya 

 is of a greyish dark-blue colour, compact, and with an even fracture. No mineral is 

 discernible by the naked eye. With the microscope it seems to be allied to felspathic 

 basalt. Grains and crystals of olivine, already changed into a yellow substance on the 

 edges, are, with magnetite, the most conspicuous elements ; they are enclosed in a 

 network of minute crystals of plagioclase and augite. Small veins, lined or filled with 

 zeolites, traverse the rock. 



In this locality other basaltic rocks were collected which must be classed with the 

 dolerites. They are remarkable for the large dimensions attained by the crystals of 

 augite, which often measure more than a centimetre. Under the microscope the 

 outlines of the sections of augite are very distinct, and show that this mineral is 

 perfectly developed on all its faces. It is often twinned according to the ordinary 

 law oo Poo ; at other times the crystals cross each other in such a way that the traces 

 of the faces r rr' form an angle of about 80° ; in this case all would seem to indicate 

 that the crystal is twinned following the dome -Poo. Some crystals of pyroxene 

 are zonary, and possess the hour-glass structure ; some of these have an internal 

 structure which only shows itself with polarised light. A section with irregular 

 outlines shows strise in connection with the zones of growth ; this section is traversed 

 by a series of parallel lines corresponding to the prismatic cleavage. It may be seen 

 between crossed nicols that it is traversed by three series of lamellae, of which one is 

 almost perpendicular to the direction of the cleavage, the two others being perpendicular 

 to one another, and making an angle of about 45° with the first. The pleochroism is — 



/3 > y > a 



reddish. violet. yellowish. 



