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



tributed in the rock ; they are zonary, and almost always show numerous fine lamellae 

 twinned according to the albite law ; periclinic lamellae are also sometimes seen. Other 

 sections of triclinic felspar appear to belong to microcline or microperthite ; they are 

 slightly milky plates, in which some more or less lenticular intercalations of another 

 felspar appear, resembling the inclusions of albite in microcline. Orthoclase appears in 

 nearly opacpie milky sections, rarely twinned according to the Carlsbad law, but, on 

 the contrary, almost always forming a single crystalloid without interpositions of 

 hemitropic lamellae. The two cleavages at right angles, which characterise this species, 

 are apparent in some cases. This felspar, which seems more altered than the plagioclase, 

 shows yet no trace of decomposition into micaceous matter, nor of saussuritisation. 

 The sections extinguish uniformly. It appears probable that this mode of decomposition 

 is due to an action of a special nature. The orthoclase is often seen bordered with a 

 vitreous zone due to the fusion of the felspathic matter. Although no vitreous 

 inclusions are to be seen, the sections of felspar are riddled with air-bubbles. Quartz 

 in irregular grains is recognised by its brilliant colours in polarised light, and the arms 

 of the cross of monaxial crystals appear in convergent light. This mineral is remark- 

 ably fissured and split, being also filled with gaseous inclusions such as are observed 

 when quartz is fused in fulgurites for instance. No liquid inclusions are to be seen, 

 but some fine vitreous ones have been observed ; these are in all probability of second- 

 ary origin. This mineral is represented in the microscopic preparations by numerous 

 sections showing clearly all the characters of the species. Biotite appears in the form 

 of dark -brown strongly pleochroic sections. The outlines are irregular and black, 

 but not opaque at the edges, as is common to the hornblende of the andesites and 

 of the basaltic lavas. This mica presents no noteworthy peculiarities, except that a 

 number of excessively minute microliths of a very pale greenish colour are attached to 

 the broken edges. Some of these little prisms extinguish at angles which may rise to 

 as much as 40° ; they should be classed as augite. It is also to be remarked that their 

 long axes are arranged in directions more or less parallel to the pinacoid of the mica 

 they surround. Augite has also crystallised as inclusions in the interior of the biotite. 

 Here we have facts which bear a close analogy to what has been observed in the case 

 of the hornblende of the andesites. Everything leads to the conclusion that, in the 

 embedded as well as in the eruptive rock, the formation of the little crystals around mica 

 or hornblende must be due to the same caustic action. Irregular granules of titaniferous 

 iron, sometimes surrounded by a zone of rutile, are found in the altered granitic rock. 



Finally, we may mention amongst the ejected rocks fragments of quartzose rocks 

 which were embedded in the eruptive mass. These are milk-white in colour, and 

 extremely fine grained in texture ; they have a fritted appearance like the granite just 

 described, and they are furrowed by fissures of contraction. This appearance of the 

 specimens plainly shows that they have been submitted to intense heat. A zone of 



