26 ME. J. A. DOUGLAS (XN" GEOLOGICAL SECTIONS [April 1914, 



and showing pronounced corrosion -borders. Augite is equally 

 abundant as large pale-green phenociysts, with marked zonaiy and 

 hour-glass structure. 



The ground-mass consists of a brownish glass containing small 

 felspar-laths, magnetite, and abundant short prisms with hexagonal 

 cross-sections of a brown pleochroic apatite. The pleochroism 

 appears to be due to numerous minute inclusions arranged in fine 

 lines parallel to the c axis of the ciystals. 



Note on the zonarv structure in the augite. — These 

 crystals show a darker, more ferruginous kernel with a wide 

 extinction-angle (about 40°) surrounded by alternating light and 

 dark bands, with a paler margin having an extinction of about 32°. 

 The simultaneous ciystallization of the hornblende- and augite- 

 phenocrysts probably has an intimate connexion with the colour- 

 variation in the zones of the latter mineral. If we suppose that the 

 commencement of growth of the hornblende occurred slightly later 

 than that of the augite, on its separation the magma would become 

 impoverished in iron and the composition of the next augite zone 

 would tend to be less ferriferous and of a paler hue, while an} T 

 later change of physical conditions causing partial resorption of the 

 hornblende with liberation of iron would possibly result in the 

 formation of a more ferriferous zone in the augite. Thus successive 

 growth and resorption in the one mineral may be recorded in the 

 varying colour-zones of the other. 



The second locality where igneous intrusion occurs is situated a 

 few miles up a small tributary stream which enters the Mauri 

 Eiver from the north, about Kilometre 250 (see map. PI. X, fig. 2). 

 Here, the Mauri tuffs are penetrated by a dyke of trachyte or 

 traclry-andesite, which is of interest as showing a close similarity 

 with the rocks of the second or trachytic group of the coastal 

 series (see pp. 21-22). 



(A 2C ) Trachy-andesite ; Kilom. 250, Mauri Eiver (Bolivia). 



Macroscopic characters : A pale-grey trachytic rock, with 

 phenociysts of clear white felspar and smaller crystals of black 

 hornblende and biotite. 



Microscopic characters: Felspar, chiefly plagioclase near 

 oligoclase, in large clear porphyritic ciystals, with Carlsbad and 

 albite-twinning and zonaiy structure. Hornblende abundant 

 in idiomorphic ciystals ; strongly pleochroic, deep reddish-brown 

 to pale yellow. Biotite dark brown, in part subsequent to the 

 felspar-phenocrysts. Apatite occurs sparingly as small needles. 



The ground-mass consists of a colourless glass, with abundant 

 microlites of felspar, rather short and stout in shape, approaching 

 an orthophyrie type of structure. Lines of flow not evident. 



