HA RD WICKE S SCIENCE- G OSS I P. 



193 



NOTES ON GRANITE. 



By HENRY FLECK, F.G.S. 



OCKS are divided 

 into the two great 

 divisions, Igneous 

 and Sedimentary ; 

 the former are 

 again divided into 

 Vitreous and Cry- 

 stalline. It is to 

 the last — the cry- 

 stalline igneous 

 rocks — that our 

 attention will be 

 chiefly directed. 



The crystalline 

 igneous rocks may 

 be regarded as 

 principally sili- 

 cates of various 

 substances, such as 

 alumina, magnesia, 

 calcium, iron, po- 

 tassium, and sodium. These elements in fact make up 

 the great bulk of the earth's crust. Silica exists in 

 various proportions in these crystalline rocks ; in some 

 of them the whole of it is combined with the other 

 elements as bases ; in others, the silica is present in 

 greater quantity than can so combine, and the excess 

 remains free. Rocks with this excess of silica are 

 termed Acid ; those with the smaller quantity (60 

 p. c. and under) are termed Intermediate and Basic. 

 Granite containing, as it does, upwards of 60 p. c. of 

 silica, belongs to the acid division. It is typically a 

 crystalline-granular admixture of the minerals quartz, 

 felspar, and mica ; without any amorphous or micro- 

 crystalline matter. The felspar is the most abundant 

 mineral ; next comes the quartz ; then the mica. 

 The quartz, however, has not assumed a definite 

 crystallised form, but envelopes the other constituents, 

 and has evidently been the last to crystallise. The 

 felspar is usually white or pink orthoclase, occasionally 

 triclinic felspars, such as albite and oligoclase, are 

 present in small quantity. The mica also is variable 

 No. 273.— September 1887. 



in kind, being usually muscovite or biotite ; but 

 sometimes lepidolite, lepidomelane, or other mica. 



The felspars are such important constituents of 

 granite, that it will be advisable to devote a little 

 time to the consideration of the principal varieties. 

 As just mentioned orthoclase is the most generally 

 occurring felspar in granite ; but occasionally other 

 varieties are present. Now orthoclase is the only 

 common kind which crystallises in the monoclinic 

 system ; all the others are triclinic in form. The 

 principal means for distinguishing the triclinic from 

 the monoclinic forms are the determination of the 

 angles made by the principal cleavage planes ; and 

 the presence of fine striations on the basal cleavage 

 plane of the triclinic felspars ; these striations are 

 absent from the similar planes in orthoclase. It is 

 extremely difficult to distinguish the various triclinic 

 felspars when they occur in small crystals ; they have 

 therefore, as a matter of convenience, been grouped 

 together under the name Plagioclastic felspars ; those 

 crystallising in the monoclinic form being called 

 Orthoclastic. Chemically all the felspars are silicates 

 of alumina, with silicates of lime, soda, potash, &c, 

 or any two or three of these bases, which often replace 

 each other to a certain extent in the different species. 

 Orthoclase for instance, is a silicate of alumina and 

 potash ; oligoclase is a silicate of alumina and soda, 

 but the latter is usually replaced more or less by lime 

 or potash ; albite also is a silicate of alumina and 

 soda ; labradorite is a silicate of alumina lime and 

 soda ; and anorthite a silicate of alumina and lime. 

 These facts are shown in the Table of Felspars, which 

 also mentions the crystalline system, chemical com- 

 position of each of the five principal kinds of felspars. 

 All the species, it is seen, crystallise in oblique forms, 

 generally the triclinic. Their hardness is between 

 6 and 7 in the usual scale ; and specific gravity from 

 2 "59 to 2*78. They have all a perfect cleavage 

 parallel to the basal plane and to the shorter diagonal 

 section ; the angle between these planes of easy cleav- 

 age being 90 in orthoclase ; and varying 3 to 5 

 from 90 in the triclinic forms. In polarised light 



K 



