REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 43 



place it was associated with a columnar and zonary trachyte, but he could not make out 

 the contact. The white rock which he studied contained numerous crystals of vitreous 

 felspar, and black microscopic points. It is speckled, like the surrounding trachyte, 

 with dark grains. On examining the ground-mass with a lens, Darwin found it to be 

 earthy ; sometimes, however, it possesses a crystalline structure. On the eminence 

 called " the crater of an old volcano," it passes into a greyish green variety, which only 

 differs in colour and by being more compact. Here an insensible transition between 

 the two rocks is observable. Another variety is made up of numerous round and 

 angular fragments of the greenish rock embedded in the white matrix. Both these 

 varieties of trachyte are traversed by irregular veins which do not at all resemble 

 intruded dykes, and Darwin states that he never saw the like elsewhere. Both kinds 

 of trachyte contain isolated fragments, varying in size, of a dark scoriaceous rock, the 

 vesicles of which are filled by the white mass. This trachyte also includes large 

 blocks of dark cellular porphyry, containing many crystals of opaque white felspar and 

 altered crystals of oxide of iron. The cavities are encrusted with capillary crystals. 

 These fragments project from the decomposed rock in which they are embedded, and 

 exactly resemble the nodules of sedimentary rocks. But, adds Darwin, we know many 

 cases of pieces of cellular rock being shut up in trachytes and phonolites, and therefore 

 cannot draw as a conclusion from the facts described that these rocks were of sedimen- 

 tary origin. The insensible passage of the greenish into the whitish variety in some 

 cases, and the isolated nodules in others, may result from a greater or less difference 

 in composition. The rounded form of the blocks may be due to corrosion by the fused 

 mass in which they were stuck. He considers the veins to be due to the infiltration of 

 silica. The principal reason Darwin brings forward for believing that these earthy and 

 friable rocks are not sedimentary is, that it is extremely unlikely that crystals of felspar 

 and grains of mineral should occur to precisely the same extent in a sedimentary mass 

 as in a trachyte with which the former was associated. Besides, he observed that the 

 rock matrix showed a crystalline structure when magnified. 



After giving these details from Darwin of the appearance and occurrence of the 

 trachytes of Ascension, we shall describe the specimens of this type which we have studied. 

 As Darwin's account shows, trachytic rocks play a considerable part in the island. 

 It would not be easy to devote a special description to the specimens of each locality, 

 especially since very often — not to say always — we have no information as to the 

 definite part of the bed from which the rocks were taken, the label only bearing the 

 name of the hill. We may add that all the rocks of this kind, from whatever part of 

 the island they come, are very like each other. 



We shall accordingly describe them together, grouping the rocks according to their 

 Hthological affinities, but, in the case of those meriting special attention, mentioning 

 the locality from which the specimen came. 



