and a green material is present in varying amount. Its 

 grains are about one and a half millimetres in diameter, 

 rounded and firmly cemented. Under the microscope this 

 is seen to be composed of quartz in plates, or small inter- 

 locking grains or pseudomorphous after serpentine. With 

 this is a greater or Less amount of carbonate in bands, irreg- 

 ular patches and filling intergranular crevices. In the 

 general arrangement of the grains the rock shows an 

 absolute identity with dunite, and the minerals present are 

 all those which have been proved to be alteration products 

 of olivine. Moreover the presence of picotite is itself 

 evidence of the rock being originally a peridotite. The 

 opinion was then formed that this rock was an altered 

 dunite. This was conclusively proved to be correct by 

 the discovery in the basalt of the neck (in which silicating 

 and carbonating waters have not such free circulation as 

 in the breccia) of a boulder of dunite altered peripherally 

 into quartz rock with a minor amount of carbonate and 

 green silicate. The differences in the structure of the 

 quartz in this altered rock appears to be explained by con- 

 sidering the state of alteration of the dunite prior to its 

 Siliciflcation; the different forms of quartz represent altern- 

 atively replacements of olivine, serpentine, and talc. A 



specimen 



was chosen rich in picotite a 



green silicate, and > 



vas analysed with fo 



SiO a 



46-29 



Calculated Miner 



A1 2 3 



4-6G 



Quartz... 



Fe 3 3 



1-93 



Calcite 



FeO 



10-10 



Siderite 



MgO 



713 



Magnesite 



CaO 



9-36 



Picotite 



H 3 



•11 



H.'U'inatite 



co 3 



19-34 



Rutile 



TiO, 



■12 



Water 



Cr 2 O s 



•24 





MnO 



•19 





NiCoO 



•05 





