occurrenct: of quartz in ceylon. 171 



THE MODES OF OCCURRENCE OF QUARTZ IN CEYLON. 



By James Parsons, B.Sc, F.G.S. 



IN view of the attention which has recently been directed to the 

 occurrence of ailcient quartz implements in Ceylon, it may be 

 of interest to indicate the nature of the varieties of quartz which have 

 been found in the Island, and the special physical characteristics 

 of these varieties with reference to their mode of origin. 



Quartz may be defined as silica or silicon oxide crystallizing in the 

 trapezohedral group of the hexagonal system. Its specific gravity 

 is 2"65 ; its hardness on Mohs' scale 7 , thus easily scratching ordinary 

 window glass. It melts at a temperature of over 1,400* C, but in 

 igneous rocks it is, as a rule, the last constituent to solidify, being 

 moulded by and filling the interstices between other minerals which 

 in the laboratory have been shown to melt at lowert emperatures. 

 This reduction of the fusion point is probably due to the presence of 

 water vapour under great pressure. 



Quartz when quite pure is colourless and transparent, but when 

 impure may be found of all colours from white to black, and be 

 translucent or opaque.* 



The white colour is due, as a rule, to minute inclusion of gas or 

 liquid. Shades of green, yellow, red, and blue are due to traces of 

 metallic oxides, and when dark brown or black it has been shown 

 that it often contains organic compounds. 



When in the form of crystals, these commonly appear as hexagonal 

 prisms with pointed terminations, which approximate to hexagonal 

 pyramids. There are usually transverse striations on the prism faces." 

 The eonchoidal nature of its fracture may be well seen in any arti- 

 ficially chipped piece of crystalline quartz or flint, when it gives rise 

 to what is known as the " bulb of percussion." It has practically 

 no crystal cleavage, i.e., shows no disposition to split along any 

 definite plane in relation to its crystal outline. f 



The term quartz will be here taken to include the vitreous or 

 phenocrystalline forms, to which it is popularly and perhaps correctly 

 confined, as well as the cryptocrystalline or chalcedonic forms, 

 including chalcedony and impure forms, such as flint, chert, and 

 jasper. There is some question whether the cryptocrystalline form 

 should be considered hexagonal, part at least of the silica in chalce- 

 dony appearing to be triclinio. 



* Clear quartz is known in Sinhalese as palinguwa and the white translucent 

 variety as tiruwana. 



t As shown later a schistosity or slaty f^leavage may he developprl in quartz 

 masses. 



2b 7(12)08 



