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GRANITIC EOCKS. 



219 



Wall-cases 6 and 7. 



Igneous Rocks of various Kinds. 

 Preliminary Remarks by A. C. Ramsay. 



The rocks in these cases are all usually considered ig- 

 neous. They are arranged lithologically, or according 

 to their structure and composition, and without reference to 

 their geological ages. As near as possible, they follow each 

 other so as to show the manner in which igneous rochs merge, 

 or shoio a tendency to merge, into each other. 



Nos. 1 to 4 show familiar examples of the minerals of 

 which granite is composed, viz., quartz, felspar, and mica. 



Nos. 5 to 25 exhibit different varieties of granite, a 

 ternary (or triple) compound of the above minerals. The 

 specimens from 5 to 9 are grey granites, and show a passage 

 from a largely crystalline kind to the finer grained varieties. 

 Nos. 10 to 21 are reddish granites, the red tint being due to 

 the colour of the felspar. They also show similar differences 

 in the size of their component crystals. 



Nos. 22 to 25 are other ordinary varieties. Nos. 26 to 

 34 are from the outer portions or margins of certain graniti 



masses, and contain a fourth mineral, 



1C 



( 



224), which also enters into the composition of many of the 

 granitic porphyries and other specimens. From 35 to 56 

 all the specimens are schorlac eous , and exhibit the gradual 

 increase of that mineral, till, from 59 to 63, it predominates, 

 and they are termed schorl-rock. 64 and 65 are pure schorl, 

 and 66 shows the manner of its occurrence in a vein in 



granite. 



61 



to 74 are from granite veins or dykes, most ol 

 which traverse other masses of granite. They are generally 



fi 



- 7 .„ VKJ >j">j, uli^±l KsijoiaLs art? small. 



Nos. 75 to 114, with a few exceptions, are chiefly from 

 ^Ivan dykes and dykes of felspar porphyries, and other rocks 

 of like nature. Many of these, if not true eremites, are of 



Upper 

 Gallery. 



Wall-cases 

 6 and 7. 



