Upper 

 Gallery. 



"Wall-cases 

 6 and 7. 



220 



GRANITIC ROCKS. 



a granitic nature. They are generally characterised by an 



>/ 



Many of them are porphyrinic. 



Beins: 



dykes, from the smallness of their masses, they probably 

 cooled with comparative rapidity, radiating heat into the 

 rocks which they traversed. Hence their component sub- 

 stances have not had time to crystallize out separately in 

 the manner of those in true granites, although their general 

 composition is the same. 



115 to 118 are specimens of granitic veins, traversing 

 other larger grained granites, and slates which are invaii- 

 ably altered at the points of junction. (See pp. 97 — 99). 



120 to 123 are specimens of decomposing granite and 



felspar trap, from the felspar of which china clay is derived. 



(See porcelain case near case 3, in the floor below.) 



124 to 139 are felspathic traps of various kinds, 

 of them are uncrystalline, as, for instance, 125 ; others are 



Most 



( 



Some are exceedingly 



felspathic, like 126 j and others contain much associated 



silica, like 125. 



142 and 144 to 146 are syenitic granites \ that is to say, 

 they contain a little hornblende, in addition to the other 



minerals. 



143 is a schorlaceous syenite, and 148 to 152 are true 



syenites, being composed of felspar, quartz, and hornblende. 

 This prepares the way for a passage into hornblendic green- 



stone, by the disappearance 



fi 



quartz. 



Typical hornblendic greenstone consists of felspar and horn- 

 blende. Nos. 155 to 174 show these minerals distinctly 

 crystallized in the rock. From 175 to 214 most of the 

 specimens are fine grained, that is to say, the crystals are 

 either very small, or else they present no appearance of 

 crystallization at all. 215 to 217 are hornblende rocks, 

 being formed entirely of hornblende. 



220 to 235 are from Devonshire, and placed so as not to 

 separate them from the Cornish and Devon series above. 

 They are otherwise not especially connected with the pas- 

 sage of the various kinds of rock into each other. They 





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