8 



THE VOYAGE OF H.M.S. CHALLENGER. 



Fig. 4. — Augite-andesite near Fuente Pedro. 



Crystal of sanidine and small crystals of plagio 

 clase, bent and fibrous in the ground-mass. 



parried by irregular fractures more or less perpendicular to the length of the crystal 

 (see fig. 4). Augite can only be recognised in the larger crystals ; these alone have 



withstood the pressure. This mineral is polysyn- 

 thetically twinned ; it is generally in a fragmentary 

 condition, filled with fissures. Isolated hornblende 

 cannot be found, but some rather large patches of 

 magnetite surround the small sections of horn- 

 blende, forming a kind of zone, and accompanied 

 by very small scales of biotite. This seems to 

 show that the hornblende, once present in the rock, 

 has been replaced by these two minerals. We 

 ought also to mention a mineral playing a rather 

 important part in this rock, sodalite. It is seen in 

 hexagonal or quadratic colourless sections. They 

 are of primary consolidation, as is shown by their 

 forming a centre of aggregation for small crystals 

 of plagioclase, disposed as spherulites round the 

 sodalite. With the condenser a black cross is 

 rather faintly indicated; they cannot be mistaken for nepheline or any other hexa- 

 gonal mineral, because between crossed nicols all the sections remain obscured. The 

 presence of sodalite in this rock is not an exceptional fact ; it is known that this 

 mineral is found in trachytic rocks associated with sanidine and augite, in nearly 

 the same conditions as those we have mentioned. The ground-mass is formed by a 

 network of plagioclase crystals and grains of augite. The rock has a doleritic structure, 

 its mineralogical composition classing it among the augitic andesites, closely allied to 

 trachytes. 



In the same gulleys of Fuente Pedro a whitish altered rock similar to the preceding 

 was collected. The ground-mass is nearly homogeneous, and contains irregularly 

 disseminated grains of felspar ; the fracture of the rock is irregular. Under the micro- 

 scope the following microporphyritic minerals can be seen : plagioclase, sanidine, augite, 

 and hornblende. Some sections of plagioclase have exceedingly thin polysynthetic striae 

 over the whole surface. Others show the Carlsbad twinning, and also that of albite ; 

 and lastly, some show in addition the periclinic striation crossing the albitic lamellae. 

 It is rather interesting to notice that, when the sections show the Carlsbad twinning, 

 one of the two halves presents the polysynthetic lamellae of the albite law, and the 

 other half shows both the albitic and the periclinic striation. This phenomenon tends 

 to prove that these plagioclases have crystallised also according to the Carlsbad law. 

 Indeed, if we bear in mind that the periclinic lamellae ought to be seen in sections 

 parallel to x, and never in those parallel to P, and that in a Carlsbad twin the faces x 



