280 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



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 sub- 

 sequent 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 Bocks. 



Although the petrography of the volcanic series faUs 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 f- For the 

 purposes of a geologist a purely mineralogical or chemical arrange- 

 ment of rocks is singularly unserviceable. He requires to take cog- 

 nizance 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 



* Proc. Roy. Soc. Edin. vol. vi. p. 74 ; Brit. Assoc. Eep. 1867, Address to 

 Geological Section, p. 62. 



+ 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 (iiscarded as ambiguous, 

 unless we agree to use it solely as a convenient synonym for all truly vol- 

 canic rocks which are found in our Palaeozoic, Secondary, or Tertiary forma- 

 tions. 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 anytime, to avoid unmelodious repetition, the word "trap" 

 is used, it is to be taken in the sense above indicated. 



