292 C. A. SUSSMILCH AND T. W. E. DAVID. 



containing, in addition to the biotite and hornblende some 

 augite and hypersthene. The plagioclase is a fairly basic 

 labradorite, and the base is largely glassy. 



Pyroxene Group. — These may be divided into two classes 

 according to the nature of the groundmass. Perhaps the 

 more common is the pitchstone variety in which the base 

 consists of brown glass ; the other, which in hand-specimens 

 has a rather lithoidal appearance, has a holocrystalline to 

 cryptocrystaliine groundmass. The lowest flow in the 

 Belah section is a pyroxene andesite whose groundmass is 

 composed of spongy-looking felspar grains with sutured 

 junctions. Whether the cryptocrystaliine types result 

 from devitrification the evidence at present available is 

 insufficient to decide. 



The andesites are all abundantly porphyritic in stout 

 columnar felspars, and in the lithoidal variety little pyrox- 

 enes may also be distinguished in hand-specimen. Micro- 

 scopically the phenocrysts seen are basic plagioclase, 

 pyroxene, magnetite and ilmenite. The felspar is very 

 strongly zoned, the core being as basic as bytownite at 

 times. The pyroxene, which may be hypersthene or augite 

 or both, is often replaced by chlorite and carbonates. 



General Notes. — Apart from field association, the evid- 

 ence for the consanguinity of the rocks is very clear on 

 mineralogical and textural grounds, as may be gathered 

 from the foregoing notes. 



There is no doubt that the rhyolites and dacites, with the 

 intermediate dellenite and toscanites, are very closely 

 related; this is shewn by the gradual variation in the com- 

 position of the plagioclase and the varying importance of 

 the orthoclase, and by the constant presence of biotite, as 

 well as by much similarity in textural characters. 



The. dacites are connected on the one hand with the 

 rhyolite by the presence of quartz and biotite, and on the 



