S4 



Q,UARTZ. FELSPAR. 



nations, can be learned only by an examination of specimens : they 

 are, however, far from being numerous ; and a short description of 

 each is necessary in an introductory treatise. 



The most important simple minerals composing rocks are quartz, 

 felspar, mica, talc, chlorite, hornblende, serpentine, limestone, and 

 slate. 



Quartz is one of the hardest minerals of which mountain masses 

 are composed : it gives plentiful sparks with steel ; it breaks with a 

 smart stroke of the hammer; the surface of the fracture in crystal- 

 lized quartz is conchoidal, in uncrysiallized splintery : the lustre is 

 vitreous. Crystals of quartz, or rock crystals, as they are common- 

 ly denominated, have different degrees of transparency : the blue 

 varieties are amethysts. The most common forms of the crystals 

 are six-sided prisms terminated by six-sided pyramids 5 or, two six- 

 sided pyramids united, forming a dodecahedron, whose faces are isos- 

 celes triangles. Uncrystallized quartz is seldom transparent, most 

 frequently translucent, but sometimes opaque. Its colours are vari- 

 ous shades of white, grey, brown, yellow, red, and green. It yields 

 a phosphorescent light and a peculiar odour when rubbed. Quartz 

 is composed of siliceous earth, combined with a very small portion 

 of alumine. It is infusible when unmixed, but with alkalies it melts 

 easily, and forms the well-known substance called glass. It is not 

 acted upon by any acid except the fluoric. Quartz exists in veins 

 intersecting mountains, and it sometimes forms large beds, and even 

 entire mountains, which are composed of this mineral in grains call- 

 ed granular quartz, united without a cement. Fragments or crystals 

 of quartz are common in compound rocks. Grains of quartz form 

 a principal constituent part of most sandstones. The milkwhite peb- 

 bles, in gravel, are composed of quartz. Flint, chert or hornstone, 

 opal, chalcedony, and agate, are different modifications of siliceous 

 earth, which, in their chemical composition, differ little from quartz. 

 Combined with a large portion of alumine and iron, quartz loses its 

 translucency and passes into jasper, which forms beds in primitive 

 mountains, and is said to compose the substance of entire ranges of 

 mountains in Asia. 



Felspar or feldspar (a name received from the Germans) is a 

 constituent part of numerous rocks. It is hard, in a somewhat less 

 degree than quartz, and is more easily broken. It is laminar, or 

 composed of thin laminae or plates, by which it may be generally 

 distinguished from quartz. The crystals are, more commonly, four- 

 sided or six-sided prisms, whose length is greater than the breadth. 

 It has a shining lustre. The colours are white, grey, milk-white, 

 yellowish or reddish white, sometimes inclining to green. The red 

 passes through various shades, from a pale to a deep red. Crystal- 

 lized felspar is translucent. It may be melted without the admixture 

 of alkalies, and forms a glass more or less transparent, which quality 

 it derives from the lime or alkali that composes part of its constituent 



