HA RD WICKE S SCIENCE- G OS SIP. 



195 



in others again, instead of, or in addition to, ortho- 

 clase, oligoclase may be present. Or some of the 

 crystals may be porphyritically developed. In all, 

 however, quartz is present. 



The typical granite, as we have seen, is essentially 

 a crystalline-granular admixture of quartz, orthoclase 

 felspar, and mica. Porphyritic granite (or granite 

 porphyry) consists of the same minerals, but the 

 felspar porphyritically developed and the rest of the 

 rock fine-grained. Granite of this type is found 

 at Dartmoor, Shap, etc. Granitite is a variety con- 

 taining oligoclase, in addition to orthoclase. The 

 orthoclase is pink, and the mica blackish-green. 

 There is usually but little mica present, however ; 

 quartz and felspar being the principal constituents. 

 Cordierite granite, in addition to the three usual 

 minerals, contains cordierite or iolite (a blue silicate 

 of alumina, magnesia and iron). The mica is some- 

 times absent ; greenish oligoclase is often present. 



In haplite there is no mica, the rock being simply 

 a crystalline-granular admixture of quartz and felspar. 

 Graphic granite is a variety of this rock, in which 

 the quartz is developed around, and parallel to the 

 felspar, in such a manner that sections made at right 

 angles to the plates of quartz and felspar present 

 markings resembling Hebrew characters. Pegmatite 

 is a name given to another variety in which also the 

 quartz and felspar have crystallised together, and 

 in which some white mica is usually found. Luxul- 

 lianite consists of flesh-coloured orthoclase, quartz, 

 and schorl. There is no mica. This rock has not 

 been met with in situ, but boulders of it occur in 

 Cornwall. Felstone is a fine-grained rock, with 

 little, if any, mica present. The matrix is composed 

 of quartz and orthoclase, with occasionally granules 

 of plagioclase, and is imperfectly crystallised. When 

 felspar crystals are largely developed, the rock is 

 termed felspar porphyry ; or if quartz is porphy- 

 ritically developed, a quartz porphyry, or Eloan. 

 The base of granulite also is fine-grained, and con- 

 sists of orthoclase and quartz, but no mica. Several 

 other minerals are, however, often present, such as 

 garnets, schorl, and hornblende. It is usually schis- 

 tose in structure. In trap granulite the felspar is 

 one of the triclinic species, and, as is usual when 

 orthoclase is replaced by plagioclase, the rock is 

 poorer in silica. In the next rock, greisen, there is 

 no felspar, either orthoclase or plagioclase, but quartz 

 and mica (generally lepidolite) make up the principal 

 mass. Cassiterite usually is found in strings or veins. 

 Gneiss is similar to granite in composition ; the chief 

 difference between the two being, in fact, that gneiss 

 is foliated and granite is not. Gneiss may, indeed, 

 be called foliated granite. The constituent minerals 

 are arranged in approximately parallel layers, and it is 

 to this disposition of the mica that the schistose or fissile 

 character of the rock is due. Protogine and cornu- 

 bianite are varieties of gneiss. The latter is some- 

 times met with at the contact of granite with slates, 



and is granular-scaly in structure. Protogine contains 

 both orthoclase and oligoclase, as well as a variety 

 of talc. It is found amongst the crystalline rocks of 

 the Alps. Syenitic granite is granite with hornblende 

 added to the usual constituents. Syenite is granite, 

 with the quartz absent, or but sparingly present, and 

 the mica replaced by hornblende. 



Constitution of Granite and its Principal Varieties. 



With respect to the weathering of granite : it is 

 found that the decomposition usually takes place by 

 the decay of the felspars, the soluble parts of which 

 are the silicates of potash and soda. These bein? 

 removed, a large part of the alumina and parts of the 

 silica remain as hydrates, forming'kaolin — a silicate of 

 alumina. The quartz is scarcely affected by decom- 

 position ; the mica becomes soft and yellow. Granite 

 is sometimes found weathered to a considerable depth 

 — as much as twenty feet in the south-west of England, 

 where it can be dug with a spade. 



In granitic veins the rapidity of decomposition is 

 found to depend upon the crystalline condition of the 

 rock ; those parts the most perfectly crystallised under- 

 going decomposition the most rapidly. 



It may now be well to glance at the various theories 

 respecting the origin of granite. Is it of igneous, of 

 aqueous, or of metamorphic origin ? Each has been 

 claimed for it, and each denied. In a discussion of 

 this kind, a good deal of difficulty arises from the 

 uncertainty attaching to the meaning of the word 

 "metamorphic." In its strict application it means 

 simply "transformation;" but the term is loosely 

 applied, mostly without any attempt to define or 

 restrict its meaning. Used, then, in the most 

 general sense, it does not appear that there are any 

 good reasons for denying a metamorphic origin to all 

 eruptive rocks ; since the term metamorphic is not 

 used to define an extent or degree of alteration, but 

 simply to indicate or assert that change of some kind 

 or other has taken place. The crystalline schists, for 

 instance, are metamorphic — they are a changed form 

 of sedimentary rock. These may pass on, by a con- 

 tinuation of the cause — whatever it may be — that 

 produced the original alteration, through a vast series 



K 2 



