Outlines of Geology. 249 



and oxide of iron, and crystallizes in six-sided plates and prisms ; 

 it is easily recognised by its disposition to lamellar division, and 

 by the tenuity and elasticity of the plates into which it admits of 

 being thus mechanically separated. 



The characters of granite depend much upon the perfection or 

 prevalence of one or other of these ingredients. Their aggre- 

 gation in the crystalline form is, in some specimens, distinct and 

 well denned, in others imperfect, forming what are usually called 

 fine and coarse grain granite ; large crystals of felspar sometimes 

 prevail, and from these the leading hue of the granite is derived ; 

 or where mica abounds, the rock acquires a lamellar and slaty 

 fracture, and is then called gneiss; and where the felspar is very 

 sparingly disseminated, or altogether wanting, or where garnets 

 supply its place, granite is said to pass into mica-slate, and this 

 again into quartz-rock, by the partial or entire disappearance of 

 the mica. 



Hornblende is an alumino-siliceous mineral, containing mag* 

 nesia, and abundant in black oxide of iron ; it forms prismatic 

 crystals, which are sometimes blended with granite, and some- 

 times with felspar only, constituting syenitic granite and Syenite, 

 and where crystals of felspar are embedded in massive felspar, 

 quartz and hornblende being occasionally superadded, but not 

 predominant, the rock is called porphyritic, or porphyry. It is 

 right here to advert to syenitic and porphyritic rocks, as associates 

 of the granitic formation, though we shall find that there also 

 exists a very close resemblance between them and some of the 

 varieties of greenstone, included in the family of trap rocks. 

 I have also mentioned marble and serpentine, as accompanying 

 granite. (See Vol. XIX. p. 82.) The former is here distinguished 

 as primitive, in opposition to the transition marble, which abounds 

 in organic remains and lies above the slate ; the primitive is gra- 

 nularly foliated in its texture, and without any trace of fossil 

 animals or vegetables ; it is celebrated as a material for sculp- 

 ture, when white and fine-grained, and is, in all cases, a valuable 

 ornamental substance. 



Serpentine is a rock, concerning the characters and relations of 



