HARD WICKE 'S SCIENCE - G OS SIP. 



73 



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QUARTZ I ITS VARIETIES AND MODES OF FORMATION. 



By the Rev. J. MAGENS MELLO, M.A., F.G.S. 



UARTZ is in its many 

 forms probably the 

 most abundant, as 

 well as one of the 

 most beautiful, of all 

 the various minerals 

 which enter into the 

 formation of the earth's 

 rocky surface. To 

 describe it and its 

 principal varieties, and 

 to give a short sketch of the modes of its occurrence 

 and of its formation, will be the object of these papers. 

 Among the elements known to chemistry is one 

 named Silicon, sometimes called Silicium ; the oxide 

 of this substance, which is never found in a free state 

 in nature, constitutes Silica, the chemical name for 

 Quartz and all its varieties. Its pure crystallized 

 form is familiar to us as the colourless and transparent 

 Rock Crystal. 



Rock Crystal. — As Rock Crystal, the typical form 

 of quartz is a hexagonal prism terminated at each 

 end by a rhombohedron, when broken it will be 

 seen to have a conchoidal or splintery fracture. 

 Rock Crystal is very widely distributed, being found 

 in rocks of all ages. The most beautiful and perfect 

 specimens are usually obtained from large cavities or 

 geodes in the older igneous rocks, and also from 

 veins in these and other rocks. The size and colour 

 of quartz ciystals vary greatly ; some are so small 

 as to be microscopical, whilst others are of very 

 considerable bulk. In the museum of Beme may be 

 seen specimens of both the clear rock crystal and 

 also of black or smoky quartz upwards of a foot in 

 length ; there are also some very large ones in the 

 British Museum. Quartz crystals are often found 

 presenting almost every shade of colour, — yellow, 

 brown, black, red, blue, violet, and green. Various 

 names have been given to these coloured varieties. 

 The violet, blue, and some of the yellow, and even 

 of the white crystals, which, when fractured, are seen 

 to have a peculiar undulated structure, which Sir D. 

 No. 148. 



Brewster pointed out, have been classed together as 

 Amethysts, a name often popularly restricted to the 

 violet crystals, which owe their beautiful tint to the 

 presence of oxide of manganese. Violet amethysts 

 are not uncommon in the geodes occurring in 

 volcanic rocks in many localities ; but the finest are 

 obtained from Siberia, Persia, India, and Ceylon ; 

 whilst Brazil yields white and yellow amethysts. 

 The yellow and brown crystals known as Cairngorms 

 are varieties of rock crystal or of crystallized quartz, 

 if we restrict the term rock crystal to the clear 

 colourless specimens. The darker brown and black 

 ciystals, as well as those designated as Cairngoi'ms, 

 may be grouped under the common name of Smoky 

 Quartz. The dark green quartz is called Prase, and 

 is coloured by amphibole ; there is also a lighter gi-een 

 species known as Chrysoprase, tinted, it is said, by 

 oxide of nickel ; whilst oxide of iron probably 

 gives colour to the numerous red varieties. The 

 common milk-white quartz, which is the ordinary 

 quartz of veins and of quartz rock, will be found, on 

 microscopical examination, to be really transparent 

 quartz, but so full of minute cavities as to cause it . 

 to assume its milky opacity. 



Quartz Rock. — Quartz Rock, or massive quartz, is 

 often found in mountainous masses, hundreds of feet 

 in thickness. Many of the quartz schists and mi- 

 caceous schists consist chiefly of quartz irregularly 

 split up by thin leaflets of mica. 



6'(?«r/jA);/i'.— Sandstone rocks, often consisting of 

 little besides more or less rolled grains of quartz, will 

 have been derived from the breaking up, under 

 various denuding agencies, of rocks in which quartz 

 has been the prevailing mineral. 



Quartz Veins. — Veins of quartz have already been 

 mentioned. These ai^e very frequent in the old slate 

 and schist rocks, sometimes forming broad and irre- 

 gular bands ; at others, mere threads traversing the 

 other materials. Such; veins will often present 

 open spaces in which the quartz will be found regu- 

 larly crystallized. 



Flint, Chert, Horn stone. — Flint and Chert are 



