REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 21 



purplish. On the other hand, this mineral is found with the form of parallelogrammic 

 sections which have in their aspect a great analogy to bronzite ; the optical properties 

 of the mineral under parallel light might agree with this opinion, although the great 

 number of quadratic sections seems unfavourable to it. The minute size of this mineral 

 prevents its examination in convergent light, and we therefore leave it undetermined. 

 As stated before, the base has undergone globulitic devitrification. 



One of the specimens collected near Porto Praya belongs to the phonolites. This 

 rock is greenish grey, with waxy lustre, compact, with shining macroscopical felspathic 

 lamellae, which can be seen with the naked eye in the ground-mass. Under the micro- 

 scope this rock shows the structure and composition of phonolite. Rather large 

 hexagonal and quadratic sections must be ascribed to nepheline ; they are colourless, 

 crossed in the quadratic section with rectangular lines of cleavage ; the hexagonal 

 sections remain dark when rotated between crossed nicols ; the quadratic, on the 

 other hand, extinguish parallel to the sides. The polarisation colours are of a 

 bluish shade, and rather pale. The hexagonal sections show, with convergent 

 light, a very indistinct black cross ; the double refraction is negative. The lines 

 of cleavage are very marked, and are parallel to the base of the prism ; twinning 

 is never observed in them ; but the optical phenomena show certain anomalies 

 which must be due to mechanical action. This rock also contains large crystals of 

 sanidine, twinned according to the Carlsbad law ; they show the characteristic fractures 

 and extinction of this mineral. These peculiarities, but specially the optical phenomena 

 in convergent light, prevent these sanidine sections being mistaken for nepheline. 

 These sections show the arms of the hyperbola of biaxial crystals ; and, moreover, 

 a section twinned according to the Carlsbad law sometimes shows on the left individual, 

 for example, a bissectrix indicating that the plane of the optical axis is parallel to the 

 composition plane, whilst the other individual presents phenomena very analogous in 

 aspect to those of the monaxial crystals. Here, no arm of hyperbola is to be seen, but 

 as it were a very eccentric cross, whose arms are perpendicular and parallel to the 

 length. To observe these phenomena it is not necessary that the section should be 

 twinned : we see, indeed, single crystals, prismatic like those in question, some of them 

 showing the bissectrix, the others the pseudo-black cross. Rather numerous crystals of 

 titanite and lamellae of biotite are found in the microscopic preparations. It is difficult 

 to ascertain the true nature of small dichroic needles, with vague outlines and slightly 

 fibrous at both ends, which are embedded in the rather altered ground-mass. Some 

 of these needles give straight extinctions, others extinguish at about 20° ; it seems 

 probable that they belong to augite. 



We have examined some specimens of calcareous rocks found near the coast 

 at the south of St. Iago, to which Darwin devoted a very detailed description. 



