bearing on theoretical Speculations. 113 



present a trace of them, excepting in the single point in the 

 west of Pembrokeshire, which has been already mentioned ; for 

 the trap of Tortworth in Gloucestershire, though it nearly ap- 

 proaches the Bristol coal-field, yet appears to be limited to 

 the transition group. 



In the carboniferous districts our former difficulties again 

 recur in limiting the age of the associated trap. In one in- 

 stance, the Cleaveland trap dyke (in the north of Yorkshire) 

 traverses not only the coal-measures, but also the incumbent 

 red marl, lias, and inferior oolite. We must however, I think, 

 consider far the greater part of the trap rocks to have been 

 produced before the age of these formations ; for how other- 

 wise can we account for the absence of trap in these last, 

 which throughout England is, with this single exception, I be- 

 lieve total. We have here, then, in the carboniferous series, an 

 example of an intermediate group containing much less of 

 volcanic rocks than those which preceded, and much more 

 than those which succeeded. 



In the oolitic and other more recent formations of England, 

 I am not acquainted with any instance of the occurrence of 

 any rock of the trap family, excepting only the just- mentioned 

 case of the Cleaveland dyke. On the continent, however, Elie 

 de Beaumont has noticed, associated with the lias in the hill 

 of Chardonnet(Alps of Brian9on), a petrosiliceous eurite (com- 

 pact felspar?), with a little hornblende, in beds of considerable 

 extent. In the Vicentin, at Predazzo, and in the Tyrol, au- 

 gite porphyry occurs in the lias and oolite. Brongniart re- 

 fers these rocks to his entritic group, which he considers as 

 occupying a middle place as to age, between the older granites 

 and younger trachytes and basalts in the volcanic series, and 

 ranging from the transition to the older tertiary beds : this he 

 defines as composed of rocks having their parts interspersed 

 with crystals, nodules, and portions confusedly crystallized ; 

 it seems to include the rocks more commonly called trap por- 

 phyry, compact felspar with hornblende, and several varieties 

 of toadstone (variolite, spilite agatifere, &c.). See TabL des 

 TcrreinS) p. 344. 



In the tertiary period, the north of Ireland presents us with 

 an extensive area of basalt reposing on the chalk; the same rock 

 occurs superior to the scaglia of the Vicentin, associated with 

 trachyte. Trachyte and lava, evidently of more recent origin 

 than the tertiary lacustrine deposits, also abound in the vol- 

 canic districts of Auvergne,of the South of France, near Mont- 

 pelier, and Toulon, of the lower Rhine, &c. ; and it seems 

 probable that the basalt of Cassel, and even of Saxony, be- 

 longs to the same period. Indeed, basalt and trachyte ap- 



N, S. Vol. 9. No. 50. Feb. 1831. Q pear 



