ERAS OF ACTIVE VOLCANIC ERUPTION. 571 



affected by igneous eruptions trending in close parallels from N.N. W. to S.S.E., a 

 direction even at right angles to the contemporaneous line of fissure of the Breidden. 



Thus, therefore, while we are taught, that volcanic action has taken place upon one 

 and the same line of fissure during an immense succession of ages, producing lines of 

 local parallelism between deposits formed at widely distant epochs from each other, 

 we see that eruptions affecting the same deposits, have sometimes thrown the masses so 

 affected into chains having a direction completely divergent from, nay, even at right 

 angles to, each other. 



If we restrict our inquiry to the carboniferous system, in vain shall we attempt to 

 find any one direction more prevalent than another. The axis of the Glamorganshire, 

 Caermarthenshire, and Pembrokeshire coal-fields, or great coal-tract of South Wales, is 

 from east and by south, to west and by north 1 ; that of Wolverhampton from north-east 

 to south-west, those of Dudley, Lickey and Nuneaton from N.N.W. to S.S.E., that of 

 Coal Brook Dale from north-east to south-west. The strike of the beds in the field of 

 Shrewsbury is as various as the form of the promontories of the older rocks on which 

 they repose. In the Denbigh and Oswestry fields the range is north to south ; in that 

 of Bewdley Forest, and in the Titterstone Clee Hill north-east to south-west. The last- 

 mentioned district is an excellent test, since it is contiguous to the prevalent line of 

 fissure in the Silurian System, and again, like the field of Coal Brook Dale, it has been 

 equally affected with the older rocks, by the north-easterly strike ; the eruptive channel 

 of basalt by which this carboniferous tract has been penetrated being precisely parallel 

 to the axes of elevation of the ridges of Caradoc, Wenlock, and Ludlow. In fine, the 

 north-easterly and south-westerly direction so prevalent in Wales and the Welsh borders, 

 being also apparent throughout many of the superior secondary formations of England, 

 may, by some persons, be considered to indicate simply the outline of the ancient shores 

 of this island, along which the deposits have been accumulated ; and in accordance with 

 modern analogies it might be inferred, that volcanic action or elevatory movements were 

 renewed at intervals throughout many ages, upon fissures more or less parallel to such 

 lines of coast. 



Abandoning, however, this speculation, and confining our attention to the evidence 

 before us, we are constrained to believe, that although igneous rocks have been ejected 

 after long intervals, sometimes upon the very same lines of fissure, occasionally alter- 



1 The major axis of the great coal-field of South Wales, ranging from east and by south to west and by 

 north, for so great a distance as one hundred miles, and of date so clearly posterior to the old axis of the North 

 Welsh and Silurian mountains is, perhaps, the best example in England whereon to sustain that portion of the 

 theory of M. de Beaumont which implies that chains having divergent directions were elevated at distinct pe- 

 riods. (See p. 406.) I may here add that, in common with all those geologists who have laboured in the field, 

 I entertain a high respect for the talents and genius of M. Elie de Beaumont, who is one of the most en- 

 lightened modern observers j and even those parts of his theory to which I cannot subscribe, have been pf great 

 service to geologists, in pointing the way to researches and comparisons which might never have been at. 

 tempted without the guidance of his general views. 



