r)02 THE CONTINENTAL OHIGIN OF FIJI, II., 



effects will be more fully noted as the component minerals are 

 described in detail. 



Quartz is fairly abundant in lari^e ^i-ains, in some cases inter- 

 m'rown with one another. These haNe roughly parallel cracks 

 running through them and passing without interruption from 

 one grain to another, thus repeating on a microscopic scale the 

 macroscopic jointing of the rock. Breaking up of grains into a 

 mosaic is very marked. In some cases this affects the whole 

 grain, but usually it gives rise to a peripheral zone only, surround- 

 ing an unbroken nucleus. Even where the grains are not 

 •shattered, the heav}- mechanical strains they have undergone are 

 evidenced by undulose and shadowy extinction in parallel polarised 

 light, and by breaking up of the black cross in convergent light. 



Unindividualised inclusions are abundant, both gas and liquid. 

 The latter frequently show^ bubbles which exhibit spontaneous 

 movement. The arrangement of these inclusions in lines is not 

 very marked. 



Felspar is the most abundant constituent of the rock. It is, 

 on the whole, fairly fresh, though incipient decomposition is 

 noticeable, and the formation of kaolin has sometimes proceeded 

 to a considerable extent. A little of the felspar must be referred 

 to orthoclase. It is untwinned, and has a refractive index lower 

 than that of quartz and the dominant felspar. It occurs in 

 perfecth^ granular individuals of small size, scattered through 

 the rock, and is with dithculty distinguished from quartz. By 

 far the greater part of the felspar is plagioclase. It occurs in 

 subidiomorphic to granular individuals averaging abuut il'Smm. 

 in length. These have suffered considerable mechanical deforma- 

 tion, resulting in hendingand faulting of the twin lamelhe. The 

 cleavages appear as sharph^ defined cracks which the crushing 

 has rendered very numerous. 



In polarised light the very fine lamelhe, twinned after the 

 albite law, are practicall}' universal. Following the absence of 

 twinning in the orthoclase, Carlsbad twinning is ver}^ much rarer 

 than usual. Lamella?, following the pericline law, are, however, 

 relatively more abundant, giving rise to a "grated" structure. 



