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



Were it not that there are certain sections showing the characteristic cleavages of horn- 

 blende to guide us, we might hesitate in some cases to classify these green sections with 

 this species. Sometimes they may almost be mistaken for indented plates of mica ; in 

 other cases, when they are not lamellated, they are more like a mineral of the clintonite 

 group ; but the cleavages are certainly those of hornblende. Quartz has crystallised 

 last. The sections of this mineral are cracked in a remarkable way, each forming a 

 true breccia, the fragments of which are surrounded by a black border. The cracking 

 conveys the idea that the mineral has been splintered by the action of heat. Another 

 peculiarity of the quartz in this granite is the number and size of its inclusions. They 

 are relatively very large, often presenting the form of a negative crystal, containing 

 gas-bubbles and a liquid ; sometimes they contain some small well-known cubic crystals. 

 In this respect we may compare the inclusions with those of quartz in the rocks of 

 Laurwig. Enclosed minerals are rarely found in these quartz sections ; we may, how- 

 ever, mention fine needles of schorl occurring as inclusions. Finally, amongst the 

 constituent minerals there are small sections of somewhat irregular form which, from 

 the index of refraction, the colours with polarised light, and extinction, seem to be 

 zircon. 



Another fragment from the same locality is referable to granitite ; it appears to the 

 naked eye with the texture of a porphyritic granite, and the shining crystals of orthoclase 

 may attain 2 or 3 centimetres in diameter. Black mica appears scattered through the 

 ground-mass, giving it a slightly gneissose structure. As in the case previously given, 

 the large felspathic sections are microperthite ; sections of micropegmatite are occasion- 

 ally found, and more rarely the felspathic element is plagioclase, which presents at the 

 same time the twin of albite and that of pericline. The black mica is very dark biotite, 

 which often forms little nests, or the lamellae are intercalated between the various 

 constituents. Zircon sometimes occurs included in quartz, the crystals being compara- 

 tively large. 



We shall now describe another rock resembling that last treated of in some respects, 

 although the black mica is not so abundant. The texture is granitic, but the 

 specimen is much altered, being entirely penetrated by iron oxide, which gives it a 

 reddish colour. The naked eye can only distinguish felspar with rather bright cleavage 

 faces. The rock, so far as one can examine it with the microscope, is impregnated with 

 hematite, which has infiltrated into all the interstices and cleavages, and appears in 

 isolated scales of indistinctly hexagonal outline. Felspar is represented by orthoclase 

 and plagioclase. The orthoclase has often crystallised as a Carlsbad twin, but the 

 plane of composition seems to be h in place of M. The plagioclases present no feature 

 which is not common to the monoclinic felspar with which they are associated ; the 

 sections are riddled with gaseous and vitreous inclusions. The constancy with which 

 these inclusions occur in the felspar of all those rocks that have been carried along by 



