VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 133 



merits. Secondary iron ore in a fine state of division is 

 also present; this is often in the minute veins of carbonate 

 which transverse the grains in all directions. Associated 

 with the bowlingite there is a certain amount of what is 

 probably opal. The serpentine is often fibrous, forming 

 veins in which the fibres stand perpendicular to the vein- 

 walls. 



The rhombic pyroxene is considerably altered, the chief 

 alteration product being a reddish platy mineral, which is 

 probably iddingsite; bastite is also developed. Associated 

 with the bastite is a mineral which almost pseudomorphs 

 the bastite, and has pleochroism with Z = c, brownish- 

 red, and X = a, yellowish-green. This is thought to be the 

 orthorhombic amphibole anthophyllite. One other decom- 

 position product of obscure derivation, but possibly after 

 enstatite, is a colourless mineral with a high birefringence. 

 This has the appearance of talc or tremolite. The latter 

 might be supposed to have originated by pressure 1 under 

 deep-seated conditions, evidence of which is to be seen in 

 the occurrence of serpentine, or the mineral may be colour- 

 less anthophyllite, as in some of the Cornwall ultrabasic 

 rocks. The strong birefringence and the absence of pleo- 

 chroism suggest either talc or tremolite, but the actual 

 determination is a matter of difficulty. 



The diopside is very fresh and contains a few schiller 

 inclusions. 



Picotite has crystallised in the main during the early 

 stages of solidification, but one very remarkable occurrence 

 of graphic intergrowth of augite and picotite occurs. (See 

 Plate VI T, fig. 4). This particular type of structure has not 

 been encounterel by the writer in his research into liter- 

 ature on ultrabasic rocks. 



1 Cf. Flett and Hill, "The Geology of the Lizard and Meneage," p. 65. 



