64 Prof. Sedgwick on the sceneral Structure 



ft' 



age ; inasmuch as some of them are unequivocally posterior to a portion of the 

 coal measures. The discussion of these subjects would, however, lead me too 

 far from the direct object of this paper*. 



Many of the g-reat movements above described, appear to have been pro- 

 duced by an action both violent and of short duration. The nature of the 

 fractures leads us to this conclusion, as well as the nature of the mineral 

 masses resting immediately upon the dislocated beds. In some portions of 

 our coal-fields there may be indications of a passage into the next superior 

 order of formations. But in the neighbourhood of the great faults above 

 described, there is certainly no such indication, or anything whatever which 

 could mark the slow progress from one system of things to the other. 



That these great fractures were produced by some modification of volcanic 

 action is, to say the least of it, a very probable hypothesis. But, granting 

 this hypothesis, what proper measure can we set up for the intensity of vol- 

 canic power during any one geological period of the earth, except that which 

 is defined by the effects produced ? Guided by this rule we assume nothing : 

 and though we fall into error from hasty generalizations, our conclusions, 

 unfettered by theory, are ever open to correction ; and as observations are 

 multiplied, gradually approximate nearer and nearer to the truth. 



Of the cause of volcanic action we know nothing ; but we know enough of 



* At Rodham, a few miles from Wooler, the carboniferous series is separated from the Cheviot 

 porphyry by an old red conglomerate forming the base of the stratified rocks. The conglomerate 

 contains many pebbles identical in structure with the porphyry, which must therefore have existed 

 before the carboniferous series : and the conclusion is confirmed by similar phenomena in Roxburgh- 

 shire. Judging, however, from the inclination of the stratified masses, we should, I think, be 

 led to infer, that the porphyry had undergone some movement since their deposition. 



A porphyry perfectly identical with that of the Cheviots, in the same range, and sometimes 

 passing into a true granite, breaks out in Lamberton Hills, a few miles north of Berwick ; and 

 near Shiels runs down close to the coast. At this point the red sandstone (subordinate to the 

 old carboniferous formation of the Tweed), which forms the sea cliff, suddenly becomes vertical, 

 and in some places is moved more than 90° from its undisturbed position. The phenomena are 

 correctly described in a paper published by Mr. Witham in the Newcastle Transactions : but the 

 important fact of the close approach of the granitoid porphyry appears to have escaped his notice. 

 It seems to prove, unequivocally, that the Lamberton porphyry assumed its present position after 

 the epoch of the carboniferous system of the Tweed. 1 mention these apparently contradictory 

 facts for the purpose of showing, how difficult it is to prove the age of igneous rocks by any single 

 set of phenomena. 



Trappean rocks are very abundant in the higher parts of the Tweed, and have a great simila- 

 rity of mineral structure ; yet it is generally impossible to come to any certain conclusion respect- 

 ing their epoch. At Cowdenknows (a few miles north-east of Melrose), a red porphyry distinctly 

 overlies a red sandstone similar to that in the lower course of the river. 



