336 Dr. MacCulloch on ibe Geology of Glen Tilt. 



in this paper, that it passes into granite, and is associated with 

 it in an intimate manner, having no further the appearance of stra- 

 tification or of an overlying character than granite has, and bear- 

 ing precisely the same relation to the stratified rocks which that 

 does. In aspect it has generally a considerable resemblance to granite J 

 often indeed it cannot be distinguished without a careful inspection, 

 while the syenite which lies above the secondary rocks rarely has 

 a granitic aspect. This variety frequently contains mica as well as 

 hornblende, and it is indeed not rare to find the former ingredient 

 far exceeding the latter. There appears therefore a sufficient mi- 

 neral ogical distinction between these two rocks to justify us in dis- 

 tinguishing them by two names, without infringing the rule which 

 I have suggested on another occasion,* that we were not at liberty to 

 vary our principles of nomenclature, by drawing them sometimes 

 from mineralogical character, and sometimes from geological po- 

 sition. But for this purpose it is necessary that the whole of any 

 mass of rock should be considered together, and that its mineralo- 

 gical character should be defined by its prevailing, not by its oc- 

 casional composition. Whatever more experienced geologists may 

 determine on this question, it is indubitably necessary that the con- 

 fusion which I have here pointed out should be removed by some 

 expedient, and I shall gladly conform to any better suggestion ; but 

 it is too plain that the indiscriminate application of the term syenite 

 to two rocks so essentially different in connections, would tend to 

 produce a most incurable confusion in geological description. 



The last remark which I shall make on the apparent discrepancy 

 between my observations and those of the philosophers above- 

 mentioned, is also little else than a question of nomenclature. It 



* Vide Miscellaneous Paper, Vol. II. of Geological Transactions. 



