Afje of the Alkali Rocks. 161 



Bay, the latter being the outcrop furthest to the S.W., the Kettering 

 dyke the outcrop furtliest to the N.E. in the area examined. 



Microscopic Characters of the Alkali Porphyries. 



A section kindly lent me by Mr. Twelvetrees, labelled Foyaite 

 porphyry, Little Oyster Cove, contains as phenocrysts, dark-green 

 pleochroic hornblende, pale-green augite in smaller prismatic 

 crystals, plagioclase and small crystals of sphene. The ground- 

 mass consists of small rectangular crystals of alkali felspar. 



A section from the central part of the dyke at Kettering, Little 

 Oyster Cove, shows that the rock has been considerably altered by 

 weathering. The phenocrysts consist of Hornblende, more or less 

 completely altered to aggregates of micaceous and chloritic material, 

 altered plagioclase, with a ground-mass of small rectangular crys- 

 tals of alkali felspar. Another section of the same rock at the 

 contact with the diabase shows a definite, fine-grained selvage, con- 

 sisting of a dense felspathic ground-mass, in which are set fairly 

 fresh phenocrysts of plagioclase, and somewhat altered green horn- 

 blende. The hornblende in this rock is probably a soda hornblende, 

 and the abundance of felspar, particularly of the alkali felspar of 

 the ground-mass, shows that it is an alkali porphyry of inter- 

 mediate composition. Somewhat noteworthy is the abundance of 

 plagioclase phenocrysts. 



With this rock may be compared those from Woodbridge and 

 from Petchey's Bay. The Woodbridge rock in section shows as 

 phenocrysts dark-green hornblende, pale-green alkali-augite, abun- 

 dant plagioclase, and small sphenes set in a ground-mass of rectan- 

 gular alkali felspars. 



The Petchey's Bay porphyry in section has large phenocrysts of 

 plagioclase, some of which may be anorthoclase, green aegirine- 

 augite, and small sphenes in a ground-mass of rectangular alkali 

 felspars. 



There can be no doubt of the great general similarity of these 

 three rocks. In each, the same ground-mass of alkali felspar is 

 present, and the plagioclase phenocrysts predominate. Probably 

 the Petchey's Bay rock, owing to the abundance of aegirine-augite 

 is the most highly alkalic, the Woodbridge rock intermediate in 

 alkali content, and the Kettering dyke somewhat less alkalic. 

 Among the Port Cygnet rocks occur many whose texture and com- 

 position differs considerably from these three described above, but 

 in the series near Port Cygnet various alkali porphyries are present 



