REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 53 



of fumaroles. The altered surfaces are sown with extremely brilliant little black 

 crystals, standing out in relief, and never forming a part of the ground-mass on which 

 they are set. They are found in every hollow, but never on a freshly broken surface. 

 These crystals are never more than 1 or 2 millimetres long ; several individuals are often 

 united with the axes parallel ; often also they are hollow or present a skeleton-like 

 appearance. Microscopic examination shows that the dominant faces are oo P, which 

 are usually relatively well developed ; indications of oo P oo , co 5 oo , P, OP are also seen. 

 The angle of the prism mm measures 124° 30'. With the microscope a well-marked 

 cleavage following the faces of the prism may be made out, and when the crystals are 

 broken the very elongated prismatic solids of cleavage present the same angle of 124° ; 

 these little prisms have a maximum angle of extinction of about 15°. Although only 

 slightly transparent, the crystals show a perceptible pleochroism ; the light ray 

 vibrating parallel to c is of a more or less deep green, that perpendicular to this 

 direction being reddish green. These details prove beyond doubt that the crystals are 

 hornblende, and that they must have been formed by sublimation like their congeners 

 of Vesuvius, which, as described by Vom Path, 1 are in every way similar. No true 

 amphibolic trachyte has been found amongst the specimens from Ascension. The rock 

 just described is only a transitional type, and the same may be said for the next speci- 

 men to be considered. This rock, from a quarry near Georgetown, is an augitic 

 trachyte passing into amphibolic andesite. To the naked eye it hardly differs from the 

 common trachyte of the island ; it has the same greyish colour, but is perhaps a little 

 more scoriaceous, as indicated by a certain roughness to the touch. The microscope 

 shows a ground-mass composed of microliths of felspar and minute corroded crystals of 

 hornblende, showing the characteristic cleavage, brownish green in colour and dichroic. 

 Small augites, extinguishing under a high angle, and with the usual appearance of this 

 mineral in the trachytes of the island, also occur, and magnetite is a somewhat frequent 

 constituent. Sanidine in large sections is the principal mineral constituent ; the 

 crystals, which have an undulating extinction and are corroded, occur in groups and 

 twins as described in the case of augitic trachytes. Finally, there are some finely 

 striated fragments of plagioclase, occasionally twinned according to the Carlsbad or 

 Baveno law. The presence of plagioclase indicates a transition from the series of 

 trachytes to that of andesites. 



Pyroxenic trachyte passes in some cases into rocks in which the siliceous element is 

 isolated, and this forms a transition to rhyolite. 



A specimen from Red Hill (?) is an example of this transition. It is bluish grey, 

 spotted with black, and contains lamellae of sanidine 3 or 4 millimetres long amongst a 

 compact crystalline ground -mass. ' This mineral appears arranged in parallel lines ; 

 indeed, only the large shining face of a cleavage plane, parallel to M, is to be seen there. 



1 Miueralogische MittheiluDgen (JPogg. Ann., Ergiinzungsband vi., p. 198, 1871 ) 



