REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 55 



mixture of small microliths of felspar and quartz, such as is often observed in certain 

 porphyries and rhyolites. The pseudo-spherulites have a black opacpae centre, com- 

 posed of a reddish or greenish non-transparent material, which assumes a more or less 

 starlike form, and underlines the fibres of colourless minerals forming the radiated 

 aggregate. The dark substance of the spherulites may be related to certain rather rare 

 small pleochroic sections which possess some of the properties of hornblende or of 

 biotite. Perhaps hornblende, now decomposed, formed at one time an integral part of 

 the rock. 



Rhyolitic tufas also occur in the island, but amongst the specimens of rocks 

 from Ascension which we have examined, only one belongs to this type. To the 

 naked eye it exhibits a number of bluish grey, zonary, slightly schistoid splinters, 

 embedded in a pretty homogeneous mass. Under the microscope the rock appears like 

 a breccia of volcanic fragments cemented by chalcedony, or, in some cases, by hyaline 

 quartz. The fragments are angular and irregular in form, as if crushed ; they are 

 essentially vitreous, and contain felspathic microliths, which are so minute that the 

 species cannot be established except in rare cases when microlithic plagioclases are 

 observable. The spherulitic structure, to be seen in certain cases, also confirms the 

 reference of these fragments to rhyolite. In the centre of the spherulites, or following 

 the radii, there is a black opaque substance like magnetite, trichitic rods of which may 

 be seen scattered through the whole ground-mass, and giving it a blackish tint. Like 

 a great many of the rocks of Ascension, this tufa contains scales of hematite. The 

 cement uniting the fragments is siliceous ; in polarised light one sees that the quartz 

 forms a brilliant mass of grains bordering and planted on the sides of the lapilli. These 

 grains fill up the gaps, and when the space is not quite filled up by them, it forms a 

 geode, in which crystals of quartz, with faces of the prism and pyramid, may be 

 distinguished. 



Finally, we shall consider a tufaceous rock from Dry Water-Course. This tufa is 

 shown by the microscope to be composed of fragments of different kinds of rock, all 

 belonging, however, to types which are represented at Ascension. These splinters, or 

 lapilli, have been embedded in a more acid vitreous mass, showing fluidal structure and 

 of a yellowish colour, which, penetrating the interstices between the fragments, 

 corroded them. The large crystals of sanidine are rounded at the edges ; the augite 

 seems to have been entirely fused. Spherulites are visible in the vitreous substance ; 

 the silica has been subsequently infiltrated. There is enough quartz in the ground- 

 mass to justify the name of rhyolitic tufa which we apply to this rock, but there is 

 also silica of secondary origin, which has penetrated the crystals of felspar ; they 

 appear in polarised light as a mosaic of quartzose grains. 



