Dr. Mac Culloch on the Geology of Glen Tilt, 335 



more characteristic beds of quartz rock with which it is as- 

 sociated. 



The next remark I shall make is on the term syenite, which in 

 the paper alluded to is applied to that rock which I have called 

 granite. The difference here is merely a question of expediency, 

 and I shall content myself, in addition to what I have already said 

 on that subject, with a brief statement of my reasons, as I shall 

 have occasion to discuss this point at some length in giving an ac- 

 count of the Western islands. 



Werner has described syenite as an overlying formation, and 

 incumbent on granite. With a reek of these characters I have no 

 acquaintance, and his description is therefore unintelligible to me 

 in a practical view. 



But there are in Scotland (and elsewhere) two compound rocks 

 formed of the ingredients described in his definition, namely, 

 quartz, felspar, and hornblende, but occupying two positions, in a 

 geological view most distinct, and neither of them agreeing with 

 that which he has assigned for his syenite. That to which 

 I have thought it expedient to limit the term, is incumbent upon 

 the secondary strata and at the same time interferes with them in 

 the same way that certain varieties of trap and porphyry, with 

 which it is intimately connected, are known to do ; examples of this 

 are to be found in many of the Western islands. It is therefore 

 not only posterior to these strata and distinct from the syenite of 

 Werner in its geological relations, but it differs from it in compo- 

 sition, inasmuch as it does not contain mica, unless, as some also 

 of the traps do, accidentally. 



The other rock which agrees with the mineralogical definition of 

 syenite, is found connected with granite and consequently subja- 

 cent to the most ancient stratified rocks. I have attempted to shew 



