REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. & 



and P, for the two individuals, are placed side by side, we must admit, in order to 



explain the fact just mentioned, the existence of a Carlsbad twin : sections parallel to P 



are also parallel to x, and can only show periclinic lamellae in the individual of which 



the face x has been cut. Symmetrical extinctions measured in the zone P:k have 



given, for several cases, values of about 5° ; the sections showing the periclinic striae 



extinguish at angles of 12° to 14°. This seems to show that we are dealing here 



with an isomorphous mixture approaching that of oligoclase. There are also small 



sections of felspar, the physical characters of which contrast with those that we have 



just described. They are more corroded, have less regular outlines, and are crossed 



by fractures ; polarised light shows that they have crystallised according to the 



Carlsbad law, and do not show the polysynthetic striae. The angles of extinction are 



usually rather large, but at the same time it is noticed that the outlines are feebly 



marked ; this shows that these sections belong to a zone intermediate between the zones 



P : M and h : M, closer to the latter. The frequency of these larger extinctions would 



prove that this felspar is more developed in the direction of the vertical axis than 



in the direction of the edge PjM. The crystals of hornblende have a very broken 



appearance, and are greatly elongated. One cleavage predominates, and irregular 



fractures are observed nearly perpendicular to that direction ; these fractures are 



caused most likely by mechanical action due to the contracting mass, giving rise to 



deformation in all the constituent minerals of the rock. The polarisation colours of 



the hornblende sections are orange, and show that decomposition is taking place. 



Extinctions of 4° or 5° have been measured ; pleochroism is very marked. 



y > (3 > a 



dark brown yellow brown pale yellow 



The sections perpendicular to the axis c are very rare and ill-defined, as may be 

 expected from the very prismatic form of hornblende in this rock. Augite is more 

 plentiful than hornblende ; it is elongated like the latter, and often shows twinning 

 according to the usual law ; sometimes the sections are polysynthetically twinned, 

 symmetrical extinctions on both sides of the twinned lamellae measured 38°. This 

 augite is not pleochroic ; yellowish spots are seen in the interior of the section, 

 indicating incipient decomposition. Like the hornblende, this mineral often shows 

 fractures and crevices caused by mechanical action. Numerous grains and crystals of 

 magnetite are often accumulated in certain points. The ground-mass is composed of 

 an aggregation of small elongated felspar crystals, interwoven in all directions, and 

 very small augitic sections are wedged between them. The felspar microliths ought 

 to be ascribed, like the microporphyritic individuals, to sanidine or to a plagioclase 

 with small extinctions, twinned according to the Carlsbad law and that of albite. 

 The small felspars of the ground-mass, which we also ascribe to sanidine, show the 

 Carlsbad twinning without any trace of plagioclastic striae. The features of the 



(PHYS. CHEM. CHALL. EXP. PART VII. — 1889.) 2 



