178 PETROLOGICAL NOTES ON SOME OF THE ERRATICS 



Hornblende is in idiomorphic prisms with an extinction angle of 17° on (010) : 

 a = yellow ; fj = dark brown ; C = dark brown. 



a«b>c 



The sections are somewhat chloritised. The mineral occurs in one generation 

 only. 



Chlorite occurs in small flakes and also in large patches (penninite). These large 

 patches are all apparently in secondary spaces, and are associated with other secondary 

 minerals. The smaller flakes suggest original biotite, but none of the latter mineral 

 has survived. 



Augite occurs in two generations. The individuals belonging to the second are 

 about equal in size to those of felspar and hornblende. They occur in quite colourless 

 prisms with extinction angles up to 45°. Sometimes the ends of the prisms are frayed 

 out and uralitised ; the central portions are fresh. In many instances they have been 

 converted into green reedy hornblende, quite distinct from the primary mineral. The 

 augites of the first generation are in grains up to 0'5 mm. by - 3 mm., colourless and 

 with 2V very large. There is much epidote (pistacite) in relatively coarse-grained 

 aggregates up to 1 mm. in diameter, associated with calcite, uralite (in fibres), penninite 

 and a biaxial zeolite, apparently stilbite. No fresh iron ores are present, but there is 

 much leucoxene. 



372 (P. 112). Porphyrite (Plate I, Fig. 5) 



Macroscopic Character s. — Mottled rock with dark grey lithoidal base. The 

 phenocrysts are white plagioclase up to 10 mm. by 4 mm. (but mostly much smaller), 

 sharply defined hornblende prisms up to 4 mm. by 1 mm., and biotite up to 2 mm. by 

 1 mm. 



Microscopic Character s. — The rock is porphyritic with a panidiomorphic 

 granular base, whose average grainage is about 0*04 mm. The base consists chiefly 

 of acid felspar (probably anorthoclase), with small quantities of quartz, and 

 flakes of green and brownish minerals, including apparently both hornblende and 

 biotite. 



The most abundant phenocrysts are plagioclase in thick tabular crystals, giving 

 sections up to 3 mm. by 1 mm. These are zoned, and measurements indicate a com- 

 position of Ab 5 An. ( for the outer zones, while the bulk of each crystal is slightly more 

 basic than AbjAiij. These felspars are very slightly decomposed and contain liquid 

 inclusions with moving bubbles. 



Hornblende is present with prismatic habit, sharply idiomorphic and characterised 

 by a strong development of the orthopinacoid (100). The highest extinction in sections 

 from the prism zone is 15° measured from the trace of the vertical axis. The pleo- 

 chroism is: 



a = light yellowish-green ; 

 fj = dark clove- brown for the inner portions; 

 t) = very dark brownish-green for the outer zone; 

 C = very dark green, almost opaque. 

 Absorption = a < f)< C. 



This zoning is not prominent in vertical sections, but is well marked in those which 

 are cut transversely. The characters of this hornblende suggest a relationship between 

 this rock and numbers 441 (p. 180) and 452 (p. 179). The present rock may represent 



