Prof. Hull — The Volcanic History of Ireland. 147 



into whicli we miglit be liable to fall by observing merely the 

 greater or less proportion of igneous rocks in each case. There may 

 be formations penetrated by numerous sheets and dykes of trap rock, 

 none of which can properly come under the head of "volcanic," and 

 which, therefore, present no true marks of contemporaneous volcanic 

 action. Dykes of trap of an irruptive (or intrusive) character may 

 belong to a geological period different from that of the strata in 

 which the dykes occur ; and it is often exceedingly difficult to deter- 

 mine their true age. Even in the case of sheets of trap, unless very 

 careful observations are made regarding their relations to the strata 

 which inclose them, much caution is required in determining the 

 question whether they are volcanic or plutonic, or as the late 

 Professor Jukes used to say, " eruptive or irruptive." While no truly 

 intrusive rocks can be properly called " volcanic," it is clear that all 

 volcanic rocks are to some extent intrusive ; for wherever they occur 

 in sheets amongst the sedimentary strata, they must in some part of 

 their extension pass vertically or obliquely through the strata which 

 are in course of formation. Such truly volcanic lavas may therefore 

 present evidences of intrusive action amongst the beds which under- 

 lie them ; but we may feel sure that the beds which rest upon them, 

 belonging to the same formation, will be not only undisturbed, but 

 remain generally unaltered in physical constitution. 



But the most reliable phenomenon to guide the observer in deter- 

 mining the question whether ar not true volcanic action has left 

 its traces upon any formation, is the presence of beds of ashes, agglo- 

 merates, or other volcanic ejecta, which have been blown out of 

 submarine or sub-aerial vents, and have subsequently become in- 

 terbedded with the strata. For, while it is true that such accumu- 

 lations may not necessarily be present at all times amongst volcanic 

 rocks of different formations, they are, doubtless, very general 

 accompaniments ; and whenever they do occur, we may safely con- 

 clude that we are in the presence of truly contemporaneous volcanic 

 rocks ; that is to say, rocks which have been ejected in a liquid or 

 fragmental state during the period of deposition of the strata in 

 which they are found. 



But, before entering on the consideration of the successive periods 

 of volcanic activity in Ireland, one or two observations regarding 

 the use of the term " volcanic " may not be unnecessary, in order to 

 guard against misconception of my meaning. The term is generally 

 exclusively applied to designate those rocks, consisting of various 

 kinds of lava, ashes, scorise, etc., which are now from time to time 

 being poured forth or blown out of active volcanos, or those which, 

 like Auvergne and the Eifel, have become extinct in Post-Tei'tiary 

 times. We have, therefore, active volcanos and extinct volcanos, 

 with their respective lavas, etc. ; but there seems no good reason 

 why the term should not also be applied to those products of internal 

 igneous action which have been erupted during the formation of 

 Cainozoic, Mesozoic, and Palaeozoic rocks, and which are found inter- 

 stratified with them. And as there are at the present day both sub- 

 aerial and submarine volcanos, so, doubtless, there were in the past ; 



