504 THE CONTINENTAL ORIGIN OF FIJI, II., 



When the felspar is considerably decomposed, the liquid dis- 

 appears and the cavities become indistinct among the decomposi- 

 tion products. Though still fairly fresh, the felspars have 

 suffered a certain amount of decomposition. This has given rise 

 to a fair amount of kaolin in tufts and grains. The alteration 

 has in some cases gone on most rapidly along the cleavage cracks, 

 but is by no means confined to them. Cracks are common in the 

 mineral which owe their origin to the increase in bulk of the 

 materials during the process of alteration and hydration, and 

 such cracks are injected with the resulting products. The kaolin 

 is especially abundant along the(llO) parting in sections parallel 

 to (010). In addition to the kaolin, there are developed, here 

 and there, small quantities of calcite, and light yellowish-green 

 epidote. These are entirely confined to the central more basic 

 portions of the felspar. The epidote occasional!}^ forms small 

 crystals, but is usually in the shaj^e of irregular grains set in a 

 matrix of lower refractive index, composed largel}' of calcite. 



Of the ferromagnesian minerals biotite is the chief. It occurs 

 in large irregular patches about 1'75 mm. diameter. These 

 exhibit in a very marked manner evidences of the intense strain 

 to which the rock has been subjected, in the bending, faulting 

 and fraying out of the plates. Colour and pleochroism are quite 

 normal, n = golden-yellow, b = f = very dark brown. 

 Absorption : c = b»a. 

 When the principal plane of the polariser is parallel to the 

 cleavage, the mineral is practically extinguished. 



The biotite has suJBfered considerably from decomposition. In 

 almost every section parts of the edge have become greenish in 

 colour, with loss of intensity of pleochroism, though the latter is 

 still strong in grass-green and straw-yellow tints. The double 

 refraction of this decomposition product is very weak indeed; in 

 fact, some sections are practically isotropic ; most show the 

 characteristic azure interference tints of the chlorites. 



All stages of alteration can be traced, from a mere slight 

 bleaching of the biotite to a mass of chlorite, often somewhat 

 fibrous in structure. 



