12 ME. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [April 1914* 



composed ma}^ be described as a compact porphyritic enstatite- 

 andesite, varying from the centre of the individual masses, where 

 it has a black basaltic appearance, to the exterior, where the ground- 

 mass is of a greyish colour with conspicuous porphyritic crystals 

 of clear plagioclase (showing lamellar twinning) and black 

 pyroxene. This change of colour, which is often abrupt, appears 

 to be in some degree due to weathering (though the constituent 

 minerals are remarkably fresh), but chiefly to the segregation of the 

 iron -ores towards the centres of the pillows. Small amygdules of 

 calcite and silica, both crystalline and amorphous, are sometimes 

 present. (Specific gravity = 2*74.) 



Microscopic characters: Phenocrysts of plagioclase, with 

 rhombic and monoclinic pyroxenes, in a glassy ground-mass. 



The plagioclase occurs in large, idiomorphic, tabular crystals 

 showing Carlsbad and polysynthetic twinning and occasional 

 zonary structure. In a specimen taken from near the exterior of 

 a pillow (Aj) it is slightly weathered ; but, in one nearer the centre 

 (A 2 ), it is remarkably clear and fresh, though sometimes containing 

 inclusions of the ground- mass. The refractive index is distinctly 

 higher than that of Canada balsam, and the extinction-angles on 

 sections cut perpendicular to the albite -lamellae range up to 30° 

 (labradorite). 



The pyroxene -phenocrysts are represented by both rhombic 

 and monoclinic varieties, pale green to colourless, the former (near 

 en stati te) showing very faint pleochroism and low double refraction. 

 It is sometimes intergrown with monoclinic augite, or the latter 

 may occur in the form of a thin secondary marginal growth 

 round a rhombic crystal. In the more weathered example (A x ) 

 such a margin frequently remains, Avhile the rhombic interior has 

 been completely converted into a fibrous network of serpentine. 



The ground-mass is hyalopilitic, consisting of brown isotropic 

 glass, crowded with microlites and skeleton- crystals of felspar, 

 pyroxene, and magnetite. In A 2 it is almost opaque in 

 ordinary light, owing to dissemination of magnetite dust, which 

 seems to be segregated towards the centres of the pillows, where 

 beautiful examples of skeleton - crystals are abundant. Many 

 yellowish-green patches occur, and between crossed nicols show 

 spherulitic structure. The felspar -microlites are frequently 

 grouped in sheaf -like bundles. Circular amygdaloids occur, 

 sparingly filled with calcite mosaic. 



(A ;! ) Dolerite ; intrusive sill, Morro de Arica. 



Macroscopic characters : A compact, greenish-grey, micro- 

 crystalline rock without phenocrysts, composed mainly of felspar 

 with black grains of pyroxene. (Specific gravity = 2*72.) 



Microscopic characters: A much -altered rock, the bulk of 

 which consists of a felted mass of felspar-laths and pale yellowish- 

 green chlorite. The felspar appears to be chiefly oligoclase, 

 with Carlsbad and polysynthetic twinning. Extinction-angles 

 ranging from 0° to 5°. 



