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



brittle and less fusible than the trachyte which encloses it. The veins vary much in 

 thickness, measuring sometimes only a tenth of an inch, at others exceeding an inch. 

 The surface is rough, and the veins are either horizontal or inclined at any angle ; they 

 are generally curvilinear, and cut each other. Being hard and compact, the veins do 

 not weather so quickly as the surrounding rock, and they frequently project for one or 

 two feet above the surface of the ground for several yards at a time. The rock com- 

 posing them is very sonorous, and vibrates when struck ; the fragments lying on 

 the ground clink like iron when thrown against each other. The shapes assumed 

 are sometimes singular ; Darwin observed a pedestal of earthy trachyte covered with 

 the veiny rock so as to resemble a parasol large enough to shade two persons. He 

 points out, in order to explain these facts, that the hill in question shows numerous 

 jasperoid and siliceous veins, indicating that in this region there is an abundant deposit 

 of silica. He admits that the rock differs from trachyte only in its greater hardness 

 and brittleness and its less fusibility, and that probably the veins originated from the 

 infiltration or segregation of silica much as oxide of iron accumulates in certain parts of 

 sedimentary rocks. 



Amongst the specimens collected by Dr. Maclean there is a fragment labelled 

 " Piece of Clinkers," 1 of which the name and all the characters correspond to Darwin's 

 description of the veins of sonorous rock of the " Crater of an old volcano." This rock 

 is entirely penetrated with limonite ; it breaks in little plates 2 centimetres in diameter, 

 with an unequal surface, which scales off, is fusible with difficulty, and resounds when 

 struck. None of the constituents can be detected by the naked eye on account of the 

 complete impregnation with iron oxide. 



Under the microscope the rock presents certain analogies to the basalts from its 

 structure, but the mineralogical composition shows it to belong to the pyroxenic 

 andesites. The ground-mass is made up of little entangled crystals of augite of a 

 nearly violet colour, with microliths of felspar and grains of magnetic iron. Embedded 

 in this there are pretty large crystals of felspar and augite. The vitreous base, so 

 common in andesites, is wanting ; but, on the other hand, there is no trace of olivine, 

 so that in spite of the basaltic appearance when under the microscope the rock is rather 

 a transition to andesite. An examination of the felspar contained in it leads to the 

 same conclusion. This mineral is twinned according to the albite and pericline laws, and 

 sometimes after the Baveno type. Sections cut parallel to M show a more basic central 

 nucleus, which extinguishes at —7°. They are bounded by a colourless zone, hence 

 the plagioclase is probably -an andesine, not a labradorite or bytownite. We know that 

 andesine is almost never the felspar of basalt, and recent optical researches go to con- 

 firm the opinion of the older lithologists, who considered it characteristic of andesites. 

 Some sections twinned according to the albite law have extinctions of which the 



1 AccordiDg to the label this specimen comes from Southwest Bay. 



