Dr. Mac Culloch on the Geology ofGiett Tilt. 337 



consists in the use of the term greenstone, which has been applied 

 by them to some of the dark varieties of the syenitic granite. It 

 has appeared to me, as I have remarked in the body of the paper, 

 that these varieties of syenitic granite were distinguishable from 

 true greenstone by the presence of quartz, and that however mi- 

 nute their texture, and apparently uniform the white ingredient, it 

 always contained that mineral in conspicuous proportion, while in 

 greenstone, a common member of the trap family, it was rarely 

 present, and might, when present, be considered as accidental. If 

 I have been mistaken in this observation, I shall consider it un- 

 fortunate that no criterion of a mineral ogical nature can be found 

 to distinguish these rocks, if indeed the presence of the occasional 

 minerals, mica, epidote, or sphene, minerals in general of a more 

 ancient origin, cannot lend us some assistance. It is so necessary 

 to prevent rocks distinct in their geological relations from being 

 confounded, that the terms which are assigned to them ought if 

 possible to be equally distinct, since the mere effect of terms, as we 

 all experience, has in every science a powerful effect in mis- 

 leading or influencing our judgments. If there really be no such 

 distinction as I have here thought probable between certain syenitic 

 granites and the greenstones of the trap family, it will then be ne- 

 cessary to have recourse to the same expedient as we use in dis- 

 tinguishing the limestones, the addition of the terms, primary, 

 and secondary ; an expedient however, which in this case it would 

 be desirable to avoid, since many geologists are inclined already to 

 distinguish by the addition of these very terms the real trap rocks 

 which are found connected with primary or with secondary strata. 



Vol. III. 2 u 



