done on board H.M.S. i Challenger. 3 613 



Darwin, I found only on the southern island. On the northern island 

 the bulk is composed of what I take to be Darwin's " yellowish harsh 

 stone, split up into numerous pseudo-fragments." These last have an 

 appearance which recalls that of weathered orthoclase crystals. All of 

 these rocks give off alkaline water when heated in the closed tube, and 

 consist of hydrated silicate of magnesia with alumina and protoxide of 

 iron in subordinate quantity. They may therefore, I think, be classed 

 as serpentine ; and the above-mentioned resemblance to orthoclase crystals 

 may perhaps afford some indication of the parent rock. Of the more 

 recent veins mentioned by Darwin, some are bordered on both sides by 

 black bands. These bands consist of a hard infusible substance. The 

 powder has a dirty greyish green colour, and effervesces with dilute 

 hydrochloric acid, leaving a brown insoluble residue. In strong hydro- 

 chloric acid it dissolves with evolution of chlorine, and the colour pheno- 

 •mena of dissolving peroxide of manganese. It was found to consist of 

 phosphate of lime, peroxide of manganese, a little carbonate of lime and 

 magnesia, and traces of copper and iron. Like the other rocks it gives 

 off alkaline water in the closed tube. 



At Fernando Noronha some of the smaller islands at its N.E. ex- 

 tremity were visited. The highest of these, St. Michael's Mount, forms 

 one of the prominent peaks which are characteristic of the group. It is 

 very steep and formed entirely of phonolite, which occurs columnar at 

 the base and massive towards the top. On the western side, where we 

 landed, the columns are inclined to the horizon at an angle of about 30°. 

 Their transverse section looks nearly square, the corners, however, being 

 considerably rounded off. The columns are for the most part slender, 

 and their mass is of a dirty green colour. In this the sanidin crystals 

 are arranged with great regularity, with their broadest faces in a plane 

 perpendicular to the length of the column. The sides of the mount are 

 covered with loose blocks of massive phonolite fallen down from above, 

 and retained in position on a very steep incline by the branches of most 

 luxuriant creeping plants. On the weathered sides of these blocks the 

 sanidin crystals, and also the hornblende ones, though in less degree, 

 project sometimes as much as a quarter of an inch. This rock possesses 

 also, in a most eminent degree, the characteristic property from which 

 it has derived its name of phonolite ; when struck with the hammer it 

 rings like a bell. The mount is cleft from top to bottom in two places, 

 nearly at right angles to each other. These clefts are filled up with a 

 hard flinty looking substance, which appears from its structure to have 

 been gradually deposited by water trickling down the sides. Its mass is 

 concretionary and sometimes foliated ; its colour is white, to yellowish 

 white or brownish yellow. It scratches glass with ease, and does not 

 effervesce with acid. Plates of 2 to 3 millims. thickness are quite 

 translucent. Heated in the forceps it does not fuse, but turns perfectly 

 white, and is then easily crumbled between the fingers. In the closed 



