Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/420

 vicissitudes of later ages, may still be gleaned at intervals over a length of nearly 800 miles. Throughout that great space volcanic activity has long been extinct, yet it remains in full force at the northern extremity in Iceland ; and we may perhaps speculate upon the possible continuity of the present Icelandic volcanoes with those which, in Tertiary times, were in action from the Irish Channel far into the Arctic Ocean.

Nor is it merely by the vast basaltic plateaux which are left that the former extent and importance of this Tertiary volcanic activity is to be judged. From the main chain of the Antrim and Hebridean basalts there diverge innumerable dykes, which are found traversing Scotland and the north of England, even as far as the shores of the North Sea. I have elsewhere* given reasons for regarding these dykes as contemporaneous with the Tertiary volcanic series of the north-west, and I shall have much to say regarding them in a subsequent paper. Taken in connexion with the great basaltic plateaux, they furnish us with evidence of a prolonged period of great volcanic activity.

2. Nomenclature of the Rocks.

Although the petrography of the volcanic series falls to be described in detail with reference to the localities where the rocks are found, some general remarks are here required, more especially regarding the nomenclature which is to be followed†. For the purposes of a geologist a purely mineralogical or chemical arrangement of rocks is singularly unserviceable. He requires to take cognizance of the geological history as well as of the composition of the rocks ; and indeed the latter branch of inquiry is chiefly of interest to him so far as it throws light upon the former. At the same time he cannot afford to dispense with the aid of chemistry and mineralogy ; and yet this has only been too frequently the case in this country, where the nomenclature of our igneous rocks remains in much the same state as that in which it was half a century ago. In the course of the researches which are to be described in this paper, I have found it of great service to keep always prominently in view the fundamental geological subdivision of volcanic rocks into Interbedded or Contemporaneous, and Intrusive or Subsequent. Each of these two series indicates a distinct variety of volcanic action, the

Geological Section, p. 52.
 * Proc. Roy. Soc. Edin. vol. vi. p. 74 ; Brit. Assoc. Rep. 1867, Address to

+ The word "trap" or "trappean" has been commonly used in this country as a general term for all these rocks. It has been employed, however, in such various significations that perhaps it had better be discarded as ambiguous, unless we agree to use it solely as a convenient synonym for all truly volcanic rocks which are found in our Palaeozoic, Secondary, or Tertiary formations. As all the rocks which I shall have occasion to describe in this series of papers are of volcanic origin — either thrown out at the surface in the form of melted lava, or as loose dust and stones, or injected into different parts of the rocks lying beneath the surface, — I shall employ the word " volcanic ; " only premising that if at any time, to avoid unmelodious repetition, the word "trap" is used, it is to be taken in the sense above indicated.