10 THE VOYAGE OF H.M.S. CHALLENGER 



small sections of augite in the ground-mass ought to be mentioned. We have already 

 stated that both this mineral and hornblende show traces of deformation by 

 mechanical action. The augitic microliths have been crushed, they have become 

 somewhat fibrous, and taken the appearance of uralite ; this fibrous structure may be 

 nearly always connected with the bends and fractures which are observed in the mass. 

 The little augite prisms are often bent and broken at the top of the curve. The broken 

 portions have become displaced, and the space between the two fragments is filled 

 with fibres which connect the disjointed portions. The greenish substance scattered 

 in filaments between the felspathic microliths of the ground-mass is probably nothing 

 but crushed and stretched out augite. Under the high powers of the microscope, very 

 small scales with extremely sharp hexagonal outlines are observed ; these lamellae have 

 a certain thickness so as to enable the edges of the prismatic zone and of the pinacoid 

 to be seen. In other cases they are more irregular and scattered all through the mass 

 of the rock. At first sight they might be taken for red hematite, but their colour is 

 rather greyish violet than red. This colour recalls that of lamellae of titaniferous iron 

 as observed in some phyllites of the Ardennes. We consider these small hexagonal 

 sections to be the same mineral ; it can be ascertained that they are monaxial. The 

 rock which we have just been describing ought to be referred to augite-andesite, but 

 the presence of hornblende and sanidine make it a transition form to the trachytes. 



On the path to the Peak, another rock was collected with a massive ground-mass, 

 black in colour, of basaltic appearance, containing large vesicles, some of which have 

 a thin coating of a zeolitic or siliceous substance. This rock ought to be classed as 

 a dolerite. Under the microscope the ground-mass is seen to be formed of small 

 plagioclase lamellae, between which are scattered microscopical crystals of augite. In 

 the ground-mass are crystals of augite and olivine 'of the first generation. Generally 

 the felspar is less developed in large crystals ; the olivine often shows sections very 

 well defined on a part of the outlines, which at other parts are broken up and 

 corroded. It does not seem probable, if we are to judge by the fluidal structure of the 

 ground-mass around the crystals, that this corrosion has been produced by the action 

 of the magma ; possibly the olivine was already in a fragmentary state before the last 

 movements of the magma, which preceded the sohdification of the rock. The olivine 

 is rather altered, and is bordered by a yellowish zone which penetrates the interior of 

 the sections. The smallest crystals of this mineral are quite decomposed ; they appear 

 as yellowish grains, and their nature can only be made out by following all the 

 phases of alteration between the larger sections, with corroded outlines, and these 

 microscopical individuals. The olivine, as also the altered augite, contains trichitic 

 skeletons and crystals of magnetite. Another somewhat common mode of decom- 

 position has been observed in this mineral ; it is shown by a fibrous structure, the 



