74 



BRITISH TRAP ROCKS SHOWN TO BE VOLCANIC. 



who now venture to dissent from the doctrines of the Scottish philosophers. No 

 individual has more advanced this branch of the science than Macculloch : few mi- 

 nutiae of chemical combination, lithological structure, or mechanical effect escaped him ; 

 and to the accuracy as well as to the number of his observations, we are indebted for 

 many of the most satisfactory proofs of the volcanic nature of the class of rocks under 

 consideration. In short, he well remarks : " It is a mere dispute about terms to refuse 

 to the ancient eruptions of trap the name of submarine volcanos, for they are such in 

 every essential point, although they no longer eject fire and smoke 1 ." Following up 

 this train of research, the same author further proved, that granite was but one term in 

 the series of igneous products, tracing with scrupulous accuracy the passages from what 

 was formerly called primitive granite, to granitoid syenite, and syenitic greenstone, and 

 thence into greenstone, basalt, and lava 2 . 



Such is a mere outline of the grounds upon which geologists have arrived at the con- 

 clusion, that trap rocks are of volcanic origin. In the course of this work, examples 

 will be first adduced of phenomena illustrative of the relations of intrusive trap ; or of 

 volcanic matter which has burst in irregular forms through sediment previously deposited \ 

 and pursuing the inquiry in subsequent chapters, it will be shown, that the greatest 

 fractures of these deposits have taken place on lines of volcanic eruption, or upon such 

 as are parallel to them, usually accompanied by considerable changes in the condition 

 of the strata thus penetrated. There is, however, another and a very ancient class of 



1 System of Geology, vol.ii. page 114. See also this author's beautiful and instructive work on the 

 Western Highlands, with plates. Classification of rocks, &c. 



2 Having alluded to the school of Scottish geologists, I have much pleasure in recording my sense of the 

 eminent services of Professor Jameson in classifying the trap rocks of his country. Imbued in early life with 

 the tenets of Werner, his first views, like those of all the pupils of that master, were necessarily opposed to 

 those here advocated ; but we must not forget that the opposing arguments of this skilful mineralogist led to 

 many of those discussions which have at length freed the subject of its obscurities ; whilst it is most creditable 

 to the candour and philosophic spirit of Professor Jameson, that being once convinced of the igneous origin of 

 trap, he joined issue with his former opponents, and has now become one of the most efficient expounders of 

 that theory. 



Dr. Ami Boue (whose indefatigable research has led him at this moment into the Servian and Turkish moun- 

 tains,) will always occupy a place in the memory of British geologists, for having at an early period applied 

 his powers to describe the mineral structure of Scotland, and to endeavour to class its trap rocks (Essai Geo- 

 logique sur VEcosse) . Nor can we forget that Necker de Saussure was educated in the same school, and by a 

 work of his youth (Voyage en Ecosse et aux lies Hebrides) gave earnest of one day maturing into the author of 

 the Regne Mineral, (Geneva, 1836). With these brief allusions to what may be termed the history of British 

 trap rocks, I ought perhaps to advert no further to foreign writers who have enlightened us by their views, 

 but my vivid recollection of the energy and talent displayed by the Prussian geologists Oeynhausen and Dechen 

 in describing many of our insular igneous products, impels me to say that the comparisons drawn by them be- 

 tween these and rocks of similar structure on the Continent are striking proofs of the value of such inter- 

 national visits. (See Karsten's Archiv ; Inseln Skye, and other works). MM. Elie de Beaumont and Dufrenoy 

 likewise having made themselves perfectly conversant with our rocks, have drawn excellent and illustrative 

 parallels. 



