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



touch, and has a very irregular fracture. The microscope shows the ground-mass to be 

 composed of a light yellowish or almost colourless base containing numerous felspathic 

 and augitic microliths, and granules of magnetite. Brownish transparent scales of 

 biotite are sometimes found. 



The crystals of plagioclase are usually zonary, and twinned according to the albite 

 and Carlsbad laws ; they are generally formed of two large individuals enclosing 

 a few extremely thin hemitropic lamellae. In some cases one of the principal 

 components, twinned following the Carlsbad law, is polysynthetic, while the other 

 is simple, and presents traces of cleavages crossing at 90°. The outlines of those 

 crystals, which are characterised by the rarity of hemitropic lamellae, exhibit a face 

 equally inclined to the traces of P and of M, which may correspond to a dome of the 

 zone P : M (n or c). Its trace makes an angle of about 45° with the traces of M 

 and of P. That this plagioclase is a Carlsbad twin may be proved by the fact that 

 in those sections where only the two principal individuals are seen, the projection 

 of the vertical faces appears in an opposite direction in the two crystals ; these twinned 

 individuals have asymmetrical extinctions : one darkens at about 40° from the trace of 

 M, and the other at 22°. The latter observation also proves that the plagioclase is a 

 Carlsbad twin and is allied to labradorite. In the zone P : Ic the angle of extinction for 

 two adjacent plagioclastic lamellae has been found to be from 17° to 20°, which confirms 

 that this plagioclase is a mixture allied to labradorite. The sections of plagioclase often 

 exhibit reentrant angles, which in ordinary light are apt to be mistaken for 

 indications of twinning, but examination between crossed nicols shows that the 

 crystals are simply grouped without hemitropy, being united wuth parallel axes. 



Hornblende plays an important part in this andesite. It has not only crystallised 

 with the faces of the prism, but the two vertical pinacoids are often represented, and 

 one of them even rather well developed. This mineral is frequently altered and 

 surrounded by a black zone of magnetite ; in other cases it is bordered by an 

 aggregation of small prisms, which are also contained in the centre of the sections. 

 This bacillary aggregate must be considered of secondary formation ; the small prisms 

 composing it are united parallel to their length, tbey are crossed by cracks parallel to 

 the base, and are almost colourless, or exhibit a greenish tint. It is not easy to 

 measure the extinction, but when this could be done it was found to be about 40°. 

 Possibly this aggregation may be made up of small prisms of augite. They are 

 arranged in such a way as to show a parallelism between their long axis and that of 

 hornblende, and seem to behave almost like the fibrous hornblende which surrounds 

 augite passing to uralite ; here this paramorphosis appears to be reversed. The 

 alteration of hornblende becomes visible not only by the zone of magnetite, or 

 the surrounding groups of augite microliths just described, but it is accompanied 

 by a development of biotite in the heart of the mineral. The manner in which this 



