•^60 Dn Mac CuLLOCH m the Geology of Glen Tilt. 



paper the aspect of a controversial essay, rather than of that to 

 which it alone pretends, a descriptive one ; for which reason I 

 have chosen to describe it precisely as it appeared to me. Where 

 my description differs from that of others, the differences will be 

 found to consist at times in a difference of opinion respecting 

 the denominations of rocks, while on other occasions they are 

 much too great to admit of their arising from a different use of 

 the same terms, or a different mode of contemplating the same 

 phenomena ; they are differences respecting facts, and are there- 

 fore the less fit subjects of discussion. 



As the local circumstances which require to be examined before 

 an adequate notion can be formed of the true structure of this in- 

 teresting place occupy a considerable range of country, and as 

 inevitable confusion would follow any attempt to describe the mi- 

 neral beds and their geological connections in their natural order, 

 on account of their perpetual interference with the geographical 

 disposition of the ground, I have chosen to adopt a geographical 

 method. In so doing I shall the more readily be understood as far 

 as the simple specification of facts goes, and those who shall 

 incline to follow me in the investigation of this spot will also be 

 furnished with a clue by which they may trace the description, aod 

 the more easily confirm or refute, as it may happen, that which I 

 shall relate. In the present state of geological knowledge, this 

 method of proceeding seems absolutely necessary. Like the detail 

 of chemical experiments it enables the reader to follow step by 

 step the appearances from which the general results have been de- 

 duced, to examine the inferences as they are drawn from the phe- 

 nomena, and finally to determine on the legitimacy of the con- 

 clusion. Such inferences or such general conclusions, as appear 

 to me to result from a comparison of these facts duly approximated, 

 livill be stated afterwards. 



