398 ARRAN. GEOLOGY. OVERLYING ROCKS. 



syenite, a substance which has given its name to a leading 

 division among the rocks of this family. As, in strictness, 

 syenite is a compound of hornblende, quartz, and felspar, 

 although, as in St. Kilda, Mull, Rum, and Sky, the horn- 

 blende is often wanting, the mineralogical character of 

 the above mentioned rock is imperfect.* 



In terminating this brief account of the varieties of 

 these associated rocks, it may be remarked, that in some 

 places, a fine breccia may be observed, consisting of frag- 

 ments of different colours, either of compact felspar, or 

 of the other analogous substances, cemented in a basis 

 of the same nature. I may also add that in some speci- 

 mens I observed distinct crystals of augit, and in others, 

 concretions of epidote entering as ingredients. 



In describing the varieties of trap, commonly so called, 

 which here occur, it is unnecessary to enter into minute 

 particulars ; as these rocks have so often already come 

 under notice. 



Among these, greenstone is by far the predominant 

 substance, but it occurs under a great diversity of aspect ; 

 appearing also to be derived from two distinct varieties of 



* The syenite of the trap family, though nominally the same as the 

 syenite found with granite, is almost always distinguished by a peculiarity 

 of aspect easier to recognise than to describe, and most frequently by the 

 nature of the felspar. In certain cases, it is true, the resemblance is 

 strong; but it is necessary, for the purposes of geology, as before 

 remarked, that the two rocks should be distinguished by appropriate 

 terms. The syenite found with granite might perhaps be conveniently 

 distinguished by the name of syenitic granite, a name, if one is really 

 requisite, sufficient to discriminate its mineralogical character, without 

 confounding its geological relations. By this simple arrangement, a con- 

 fusion to be found, even in very recent geological writings, and, in some 

 cases, generated by the ideas attached to these terms, is avoided. It is 

 unfortunate that this term should have been applied to two rocks differ- 

 ing in such important circumstances ; nor is it easy to account for its 

 choice as originally applied to the overlying rocks ; since the well known 

 Egyptian rock whence it was derived, is a granite in geological connexion, 

 and presents decided granitic characters. The observations of recent tra- 

 vellers who have visited the quarries of Syene, prove that they are situated 

 on the skirts of the granite mountains. 



