REPORT ON THE PETROLOGY OF THE ROCKS OP ST. PAUL. 5 



part to the excrements of an immense multitude of sea-birds that gather on the rocks, 

 and in part to a coating of a white, hard, brilliant material, which I shall describe at the 

 end of this report. 



Having thus summarily disposed of the facts already made known by the naturalists 

 who have visited St. Paul, I will now enter on a minute description of its rocks. It will 

 be noticed, as we proceed, that some of their characteristics have already been pointed 

 out by the authors I have quoted, but no reliable classification could be attempted until 

 a careful microscopic study and chemical analysis of the specimens had been made, for 

 which there has till now been no opportunity. 



One of the characteristics of this olivine rock is that it presents, in general, an 

 unusually fresh appearance. One would expect that a rock like this, composed of a 

 silicate which is generally found very much altered, would present some traces of con- 

 siderable change, and the more so when we consider how completely it is exposed to the 

 most powerful agencies of decomposition. Besides the force of the waves dashing against 

 these islands, to which Sir Wyville Thomson drew particular attention, the influence of 

 atmospheric agencies, at this particular point, must, owing to great and rapid variations 

 of temperature, be exceptionally great. 1 Most of the specimens show no signs of 

 decomposition, except along the crevices. Fragments broken off from hand-specimens 

 are fresh and compact, and resemble a fine-grained quartzite from the older formations ; 

 but this resemblance cannot stand the test of investigation. 



To the naked eye the massive unaltered rock appears perfectly homogeneous : it is 

 very compact, and differs from all the olivine rocks with which I have compared it, by 

 the minuteness of its grain. 2 Its colour is a blackish-grey, bordering on green, which, 

 when deep, looks perfectly black. The specimens are sometimes covered with a coating 

 of the whitish material already referred to. The naked eye and the pocket lens fail to 

 detect any porphyritic elements, though we occasionally find in them a few small black 

 irregularly-formed particles having a brilliant metallic lustre, and seldom exceeding 

 0'5 mm. in any direction. These minute particles are readily seen on fresh fractures, or 

 on polished surfaces. By their peculiar brilliancy they resemble magnetite, but, as will 

 be shown further on, they must be classed as chromic iron. Splinters from the rock 

 are translucent on the edges, and of a greenish tint. On the fresher surfaces the 

 lustre varies from sub- vitreous to resinous ; the splinters redden in the flame of the 

 blow-pipe, and are infusible. The rock is partly soluble in hydrochloric acid, as may 



1 We must observe, however, that the alterability of peridotic rocks is not perhaps quite so easy as has been 

 often represented. When these rocks are very compact and as massive as those of St Paul, for instance, they yield 

 but little to decomposition. But the fissures which furrow that rock, and divide it into blocks, more or less regular, 

 must have allowed the waves to work their disintegrating effect upon them, and have considerably reduced the 

 primitive size of the rock-mass. 



2 The rock which most resembles the one I am describing is the peridotite of Ultenthal, in Tyrol ; but the 

 specimens I have seen have a much coarser grain than the peridotite of St PauL 



