312 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



almost general in North Wales. The great eruptive masses of por- 

 phyry in the south-west of Carnarvonshire are probably referable to 

 this period, for they never fail to disturb the regularity of the planes 

 of cleavage. 



The greenstones of Rhiw, Boduan, Penllechog, &c. in the south- 

 west of Carnarvonshire, appear to be nearly of the same age with 

 the last-mentioned felspar rocks. The serpentine of Porth-din-llaen 

 seems to be of more modern date than the greenstone in its neigh- 

 bourhood. 



There are some eruptive rocks of less extent and more modern 

 date than any of those before specified. East of Bangor is a mass of 

 trap which has thrown up into a. high ridge the Ludlow rocks ; and 

 the trap dikes on the shores of the Menai, described by Professor 

 Henslow, are of posterior date to the coal-measures. 



The map which accompanies the memoir has been compiled from 

 various sources, with the addition of the author's own observations. 

 The geographical groundwork is copied from Mr. Greenough's Map 

 of England and Wales, and that map and Sir R. I. Murchison's Map 

 of the Silurian district have been freely consulted ; Mr. A. Aikin's 

 ' Tour in North Wales,* and his description of Cader Idris in our 

 Transactions, vol. ii. 2nd series, p. 273, supplied some points; but 

 the author is especially desirous of recording his obligations to Mr. 

 William Bowman for the loan of some sheets of the Ordnance Maps, 

 coloured by the late J. E, Bowman, Esq., which were principally 

 used in laying down the boundaries of the Upper Silurian deposits 

 in Denbighshire, as well as for some other points. But the author 

 is the only person who can be considered responsible for any part 

 of the Map, as he has compiled it entirely on his own discretion. 



