REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 15 



on the sections more or less parallel to M. These sections are then divided in two 

 parts, and show two series of cleavages, which join each other at an angle of about 

 52° ; a third cleavage parallel to the junction may also be observed. It is probable 

 that the crossed cleavages correspond to the traces of P, and those of the less perfect 

 cleavage to the traces of the prism. These facts would seem to prove that the two 

 twinned crystals are joined parallel to a face of the zone P:k. The oh vine shows 

 sections which are entirely transformed into red hematite, but in which the form of the 

 outlines and the cleavages are clear. The latter are observed in the greater number of 

 sections to run parallel to the base ; they are traversed at right angles by less distinct 

 lines, which would correspond to the prismatic cleavage. In symmetrical hexagonal 

 sections the acute angles are about 80°, which would correspond to the faces of the 

 dome k. It is observed that sometimes these sections are surrounded by a very distinct 

 zone of a quite colourless glass. 



A rock corning from a dyke at the south-west of the island is an augitic andesite, 

 rather rough to the touch, vesicular, in which may be seen with the naked eye 

 plagioclases and altered crystals of augite ; zeolites have formed in the cavities. The 

 rock is altered like most of those collected in this island. The mineral which plays 

 the part of microporphyritic element is the plagioclase ; it is always rather rare, 

 occurring as large isolated crystals, in which case its outlines are deficient in 

 sharpness ; they might be said to have been blunted by the action of the magma. 

 This mineral has crystallised according to the Carlsbad and albite laws ; it contains 

 numerous vitreous inclusions. The ground-mass is composed of small lamellae of 

 plagioclase and very numerous microliths of augite having a peculiar colour ; these 

 have a slightly bluish tint, and are very decidedly pleochroic ; this property is 

 observed principally in the sections parallel to cog co : in these the rays vibrating 

 perpendicularly to the length are the darkest. Considering the minuteness of these 

 microliths, often very thin, and their pleochroism as well as their peculiar tint, they 

 might be considered at first sight as allied to hornblende ; but we have observed extinc- 

 tions which exceed 40°. In transverse sections, more or less perpendicular to the axis 

 c, it is seen that they are tabular in the direction of one of the vertical pinacoids. 

 This rock is silicified ; the silica penetrates into all the interstices, and covers the 

 crystals of augite and felspar with a layer of chalcedony. This substance is distin- 

 guished from the zeolites, rather common in these rocks, by a more intense chromatic 

 polarisation, by more decided concentric zones, somewhat similar to the zonary 

 structure of the agates, and by the radiating fibres, which are very sharp, fine, and 

 acicular. 



Some specimens were collected on the road which leads to the summit of Green 

 Mountain, an eminence of volcanic origin 2482 feet in height. One of these rocks is 

 a tufa in which rather numerous small scales of black mica are seen by the naked eye. 



