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 prejudices 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 confident 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[1] and the
- ↑ I think it necessary to say that I have throughout user the term Trap as the name of a a family, including basalt, greenstone, tuf, 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 class of rocks, it is in no danger of misleading by producing any confusion of individuals, and because it was already in use as the name of many in this family without having been rigidly limited to any one species. I have also chosen the term syenite as the generic term of a set of rocks generally allied to these, and which had already been applied to that rock by Werner, excluding from this denomination the original and classical syenite, which, as well in geological connection as in mineral character is a mere modification of granite. The compound term syenitic granite may be applied to this, as I have remarked in a former paper. Varieties intermediate between common trap and syenite may be called syenitic trap.
