ARRAN. GEOLOGY. SCHIST. 353 



of the resemblance between two rocks which seem to differ 

 widely in the respective periods of their formation; 

 although even that might admit of a doubt among those 

 who consider granite, as well as syenite and trap, to have 

 been protruded from below in a state of fusion. Whether 

 or not there be any foundation for such doubts, or whether 

 any obscurity actually hangs over the geological relations 

 of the granite of this island, the question can only be 

 with propriety examined after the stratified rocks which 

 lie above it havfl been described; and to that period I 

 shall therefore refer the very little I have to offer on the 

 subject. 



THE next rock to the granite in order of superposition, 

 is the schist.*" 



* I have taken one general term for the whole of this tract of rock, 

 although presenting many varieties or species. When geological facts 

 are to be described, it is often necessary to use terras of a general nature, 

 as it is otherwise impossible to preserve a luminous order of description ; 

 the frequent interchange of mineralogical names leading to confusion, 

 and often to misapprehension. Such terms should be viewed as geolo- 

 gical, and they are thus distinguished from mineralogical appellations; 

 which are as much required when substances are to be specified, as they 

 are inconvenient when the general structure of a district is described, 

 or when geological theories are discussed. 



There are still stronger cases where the necessity of using general 

 terms of this nature for the purpose of geological reasoning is apparent. 

 Granite for example, is distinguished by mineralogists into granite and 

 syenite. In examining the geological connexions between a body of 

 granite and any neighbouring rock, it cannot affect the reasonings 

 whether, at a particular point, hornblende is found to be an ingredient in 

 that granite or not. Here the interchange of the two terms will produce 

 confusion, perhaps error; which, in this particular case, may be con- 

 siderable, since the mineralogical term syenite, implying the variety of 

 granite above alluded to, is also applied to the leading member of a sub- 

 division of rocks superior to, or more recent than the stratified rocks, while 

 granite is inferior to these. Similar confusion may occur in other cases 

 from a similar practice ; or rather, from omitting to specify whether the 

 terms used are general or particular; often perhaps from the observer 

 neglecting to investigate the connexions of the rocks which he is describ- 



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