172 SPOLIA ZEYLANICA. 



Opal is an amorphous, hydrous form of silica. It occurs in 

 Cejdon in some cherts, and occasionally forms independent masses. 

 Precious opal is not known to occur in the Island. 



Quartz is the most abundant mineral in Ceylon, as indeed it is 

 in the whole crust of the earth. Taking first the vitreous varieties 

 known to occur, these have a wide distribution in the series of 

 crystalline schists or gneiss, of which practically the whole of Ceylon 

 is composed. In the majority of these it forms an essential con- 

 stituent, and may even be present as an accessory in the crystalline 

 limestones which occur interfoliated mth the gneiss. In the gneiss 

 it occurs usually in irregular grains moulded by the other rock con- 

 stituents and interlocked with them. It may also be intergrown 

 with the felspar in micropegmatitic fashion or extended in parallel 

 rods. The characteristic colour of charnockite is largely due to the 

 dark quartz, which is an important constituent of the rock, but the 

 cause of this colour in the quartz is obscure. The different divisions 

 of the gneiss have very varying quartz contents, and bands may be 

 found ranging from almost quartz-free rocks through types contain- 

 ing only isolated grains of felspar and garnet to rocks entirely com- 

 posed of quartz. These granular quartz rocks attain considerable 

 development over the central part of the Island, forming bands which 

 may have a thickness of several yards. It is important to note that 

 the quartz rocks are simply highly siliceous foliaeof the gneiss, and 

 not quartz veins. Casual gold prospectors in Ceylon have often 

 referred to them as " lode quartz," and many assays have been made 

 of the rock, which has only ccasionally shown traces of gold, due 

 probably to the introduction of the metal by infiltration along 

 cracks. 



When unaffected by earth movements, the rock is more or less 

 friable, and consists of interlocking grains of white quartz, sometimes 

 -with occasional grains of kaolin ized felspar and garnet. Recent 

 observations have, however, shown that this granular white quartz 

 is the source of the greater part of the glass clear quartz and quartz 

 crystals which are common, but not often observed in situ. It is the 

 clear quartz derived from the granular quartz which probably forms 

 the greater part of the material from which the ancient implements 

 have been made. The rock is more readily than any other in Ceylon 

 affected by comparatively slight earth movements. The earliest 

 stage in this dynamo-metamorphism is to shatter the rock, rendering 

 it exceedingly friable. This pay be seen on a considerable scale 

 in the mountain ridges dividing Uva from the Central Province, 

 where the friable character of the quartz caused some difficulty in 

 tunnelling operations during the construction of the Nanu-oya- 

 Bandarawela section of the railway. It may also be seen about 

 Passara and several other localities. The next stage is the develop- 

 ment of a regular cleavage, which was first observed by me near the 

 BaduUa-Passara road on the flanks of Namunakulakanda. The 



