Ch. X.] ON THE PRIMARY ROCKS. 195 



in which the elementary minerals are distinct, or when they 

 contain irregular crystalline patches or veins into which they 

 graduate. This natural mode of analysis will seldom fail 

 when a series of these rocks is freely displayed. 



The granular crystalline rocks of granitic groups almost 

 universally contain felspar and quartz ; to these is very fre- 

 quently added mica, which is commonly conjoined with or 

 its place altogether occupied by talc, hornblende, shorl, or 

 some other mineral ; and according to the nature of this 

 accessory mineral, the composition of the different rocks 

 composing these groups exhibits corresponding variations. 

 Thus, the several kinds of shorl-rock in Cornwall, of horn- 

 blende rocks in Scotland, of talcose or magnesian rocks in 

 Mont Blanc and in other parts of the Alps, of micaceous 

 rocks in the Erzgebirge, and in other mountainous districts of 

 the north of Europe, all appear to arise from the predomi- 

 nance, in certain parts of these granitic masses, of the respec- 

 tive characteristic mineral. And it not unfrequently happens, 

 in extensive tracts, that this mineral does here and there 

 pass into another ; and in conformity to this change, the 

 associated rocks undergo similar alterations. For example, 

 in Cornwall, the mica of the common granite becomes talc, 

 or shorl, whence result large beds of protogine and shorl- 

 rock ; in Ireland, the mica passes, here and there, into horn- 

 blende, producing syenite greenstone and other hornblende 

 rocks ; and in Scotland, and in other countries, similar tran- 

 sitions have been observed. 



These granitic rocks, resulting from the combination of a 

 small number of crystalline minerals variously aggregated, 

 are associated together in several modes of arrangement. 

 When viewed on a large scale, each district is characterised by 

 one or more series of these rocks, disposed in distinct masses ; 

 which are divided, by the lines or seams of structure, into 

 layers and subordinate concretions, all of which possess a 

 certain degree of apparent regularity. Sometimes the various 

 kinds of these rocks occupy separate layers ; but, not unfre- 



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