BY H. I. JENSEN. 595 



in the most acid rocks frequently contain a raicropegmatitic 

 intergrowth of quartz. Intergrowths of different minerals in 

 phenocrysts, whether it be quartz in anorthoclase, or albite or 

 orthoclase in anorthoclase, are very characteristic of alkaline 

 rocks. The intergrowth may be either micropegmatitic, micropoi- 

 kilitic, graphic, or microperthitic. The anorthoclase usually has 

 the glassy clearness and cross-cracks of sanidine, but not infre- 

 quently exhibits a shadowy extinction due to ultramicroscopic 

 twinning. This mineral is equally dominant in the comendites 

 (where it has the equant, nephilinitoid habit), the pantellarites 

 (equant, or tabular, or prismatic), the trachytes (tabular and 

 lath-formed), the trachyandesites (tabular and lath-shaped), 

 phonolites (lath-shaped), and trachydolerites (tabular and lath- 

 shaped). It occurs both as phenocrysts and groundmass. 



2. Next in importance is aegirine-augite. This mineral occurs 

 as idiomorphic stunted prisms or very fine acicular rods. It is of 

 a greenish colour, but its composition and pleochroism may vary 

 within certain limits. The mineral in some rocks is an segirine 

 intensely pleochroic in colours varying from bluish-green to bright 

 sea-green, to brownish-green or greenish-yellow. In other rocks 

 it is a feebly pleochroic segirine-augite which changes from light 

 grass-green to various tones of greenish-yellow or greenish-brown. 

 The extinction angles of the different species differ accordingly. 



The segirine of the Jellore trachyte has been isolated and 

 analysed by Mr. D. Mawson, and has given the following 

 interesting result : — 



Si0. 2 



.. 49-49% 



A1 2 3 



tr. 



Fe 2 3 

 FeO 



.. 27'76 



4-71 



MnO 



• • 0-60% 



K 2 



.. 0-18% 



CaO 



.. 3-28 



Li 2 



nil. 



MgO 



.. 0-30 



Ti0 2 



.. 1-80 



Na 2 



.. 10-10 



Zr0 2 



.. 0-89 



3. The acid alkaline rocks usually contain, in addition to 

 segirine, an alkaline hornblende. Usually this is an arfvedsonite, 

 sometimes by relatively greater abundance of CaO and MgO it 

 passes into barkevicite, and occasionally b} T the displacement of 

 ferrous iron by ferric it passes into riebeckite. It has already 

 •been remarked how the soda-amphibole usually tends to form 



