Mineralogy of Sky. 53 



to be found In the very island of which I am writing, as well as in 

 many other situations in Scotland. I do not however mean to deny 

 that instances exist where a real distinction of periods in trap 

 rocks can be proved, and I have myself observed some which I 

 shall perhaps have occasion to describe at a future opportunity. I 

 am only desirous to enforce on geologists the necessity of drawing 

 their distinctions from real and not from theoretical views, and of 

 establishing criteria which are better founded, and which rest on 

 more satisfactory evidence than that produced by the mere apparent 

 or even real superposition of an unstratified above a stratified rock. 

 I am even inclined to think that at least two very distinct formations 

 of trap are to be found in the western islands of Scotland, but I 

 cannot discuss the proofs in this place, as the island of Sky has not 

 as yet produced sufficient evidence of them to satisfy me. 



Amid the doubts which prevail in the minds of others, the pre- 

 judices derived from early habitude with erroneous systems, the 

 natural obscurity of the subject itself, and the extremely inaccessible 

 nature of these rocks, I find it difficult to preserve a consistency in 

 the account which I shall give of them, and to render the details of 

 the history of this family of rocks so distinct, as I am confi- 

 dent it will hereafter turn out to be when the subject becomes 

 better known, and when nature shall have been examined by a greater 

 succession of real observers, who, however desirous of supporting 

 systems, are still more anxious for truth. 



In commencing the account of these rocks the* trap* and the 



* I think it necessary to say that I hare throughout used the term Trap as the name of 

 a family, including basalt, greenstone, tuff, amygdaloid, trap porphyry, and many 

 other varieties of rock which have as yet obtained no names, and which constitute a class 

 equally distinguished by their geological as by their mineral characters. I have preferred 

 it because as it is derived from the external outline so common and characteristic of this 



