362 



C. A. SUSSMILCH AND W. G. STONE. 



(b) Augite-Hornblende-Andesite (Porphyritic Type). 



This occurs near the Gave House as included fragments 

 and masses of varying size in the compact, non-porphyritic, 

 andesitic dyke previously described. Two phases of this 

 type occur, one characterised by porphyritic augite only, 

 the other by phenocrysts of both hornblende and augite, 

 the hornblende predominating. In all other essentials they 

 are similar, and are undoubtedly cognate, and represent 

 inclusions from the same rock mass. The phase in which 

 augite only occurs, is the one more frequently met with. 

 These rocks also outcrop at the locality marked A on the 

 geological map (Plate LV), as described in Part I, but the 

 porphyritic type from here differs so much from that occur- 

 ring near the Gave House, that it should really be called a 

 lamprophyre, and will be described separately as an augite- 

 lamprophyre. 



Megascopic Description. — The augite and hornblende 

 phenocrysts are set in a greenish aphanitic base, and have 

 an average size of 2 to 3 mm., but individual crystals range 

 up to 9 mm. in diameter. Small oval-shaped cavities con- 

 taining chalcedony and calcite may be seen in some speci- 

 mens, and where these have fallen out during weathering 

 the rock presents a somewhat vesicular appearance. These 

 cavities may represent in part an original vesicular struc- 

 ture, although some are undoubtedly due to replacement 

 of augite phenocrysts by serpentine, calcite and secondary 

 silica. 



Microscopic Description.— Holocrystalline and por- 

 phyritic with a microcrystalline groundmass, which has in 

 some sections a distinctly pilotaxitic fabric. Besides the 

 phenocrysts of augite and hornblende observable in the 

 hand specimen, some small phenocrysts of felspar also occur. 



(i.) Phase containing augite only (slide x 32). 

 Augite isa pale yellow-green to almost colourless variety, 

 and occurs in beautiful idiomorphic, well cleaved, crystals; 



