104 THE VOYAGE OF H.M.S. CHALLENGER. 



argillaceous schist. The microscope shows that these are formed of white or reddish 

 mica in lamellae or fibres having the structure of sericite. Numerous grains of quartz 

 may also be seen, and some ddbris of monoclinic and triclinic felspars. The colouring 

 matter is iron, in the state of limonite, or a graphitic material. Other schistose rocks 

 resemble true slates ; the slabs are slightly shining and blackish. In the microscopic 

 preparations only small groups or threads of quartz, and an opaque graphitic or 

 carbonaceous mass, can be distinguished, all the other elements being concealed 

 by these. 



From the same locality we may also mention a black fine-grained quartzite, with a 

 subconchoidal fracture, resembling basalt in appearance. The rock is composed in 

 greater part of small grains of quartz with irregular outlines, fragments of granite, and 

 particles of ancient volcanic rock. Besides the quartz, calcite and decomposed mica are 

 to be seen, also some grains of felspar, and very rarely epidote. 



Finally, we have to mention a grey schistoid rock in which a few felspathic grains 

 can be made out with a lens. The microscope shows the clastic origin of the 

 specimen, the cement which unites the constituent minerals being chloritic. In this 

 rock fragments of diabase with epidote, grains of plagioclase, of microline, and of quartz, 

 have been noticed. 



VIIL— ROCKS OF MARION ISLAND. 



Marion Island ' and Prince Edward Island belong to the same group. They were 

 discovered in 1772 by the French navigator Marion du Fresne, who named Marion 

 Island " lTle de l'Espe'rance," in the hope that this island should prove an outlying 

 sentinel of the Antarctic continent. In 1776 Cook sailed between the two islands, 

 and, not knowing the names given by du Fresne, called them " Prince Edward Islands," 

 which designation is still retained for the northern and the smaller of the two. From 

 that time to the present both islands have been much frequented by whalers and 

 sealers. Sir James Ross, in his Antarctic voyage, passed in view of these rocky islands, 

 and described the black volcanic peaks of Prince Edward Island. 



Marion Island, the larger of the two, and on which alone an opportunity of 

 landing was afforded to the naturalists of the Challenger, is 33 miles round ; its shape 

 is an irregular parallelogram, about 1 1 miles in length, 8 in extreme breadth, and about 

 80 square miles in area. The highest point is about 4,250 feet above the sea level. It 



1 For the natural history of this group, see Moscley, Notes of a Naturalist, p. 1GP> ; Narr. Chall. Exp., vol. i. ; 

 Buchanan, Proc. Roy. Soc, vol. xxiv. p. 388. 



