258 PROCEEDINGS OF SECTION C. 



Titaniferous Magnetite — Generally abundant in large grains 

 not only in the ground-mass but also in the felspars and pyroxenes. 

 Intergrowths of ilmenite and magnetite in parallel bands are very 

 conspicuous in several specimens. 



Mr. W. H. Hobbes has suggested that the bands of leucoxene may 

 have been formed by decomposition along the solution planes in 

 ilmenite crystals,* and as the whole crystal aj^pears sometimes to 

 change into leucoxene, and as Mr. Skey's analyses shew a large 

 quantity of iron protoxide, this may perhaps be the case. But on 

 the other hand, these iron oxides ai'e decidedly magnetic, so much 

 so that the whole rock is sometimes magnetic, and the analyses of 

 Mr. Teale and others have proved that in some European and 

 West African rocks intergrowths of the two minerals certainly 

 occur, t 



Pyrites is usually in small quantity or absent, but occasionally 

 it is abundant. In two cases I found it associated with magnetite, 

 often surrounding it ; while other magnetite grains have minute 

 particles of pyrites scattered through them. It seems evident that 

 some of the pyrites has been derived from the magnetite. 



GENERAL ACCOUNT OF THE CHANGES THAT HAVE TAKEN PLACE IN 



THE ROCKS. 



From the foregoing descriptions it may be gathered that there 

 are three distinct stages in the alteration of these rocks, due to 

 changed conditions. In a few specimens the alteration has stopped 

 before the first process was complete ; in others the first, or the 

 first and second stages have been passed through ; while some 

 have passed through all three. 



1st Stage. — The first alteration is the conversion of the anhy- 

 drous bisilicates into the hydrous magnesian unisilicatea, chlorite, 

 and bastite (part of the silica and iron and all the lime having 

 been eliminated), with the occasional formation of secondary mag- 

 netite This change has been almost universal over the whole 

 area, only two of my specimens having escaped altogether, both 

 of which are, I think, from dykes. The hypersthene appears to 

 have yielded first, then the augite, and last the hornblende, but 

 this may not always have been the case ; indeed in the specimen 

 from the north side of Coromandel Harbour the augite has resisted 

 longer than the hornblende. The chlorite was in part dissolved 

 simultaneously with its production and was d posited as infiltra- 

 tions in the ground-mass and occasionally in the felspars. Epidote 

 was formed but rarely, if at all. The apatite which was in the 

 bisilicates remained unchanged in the process. During this 



* Bull. Museum Comp. Zool. Harvard Coll. Geol. Series, ii, p. 8. 

 t Quart. Joiirn. Geol. Soc. xi, p. 650. 



