Io8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



twenty-eight miles. They attain their maximum of thickness, up- 

 wards of 5000 feet, in Aran Mawddwy, which rises from their most 

 easterly escarpment. We may therefore infer that the main vent 

 or vents lay somewhere in that direction. The noble range of pre- 

 cipices facing westwards shows how greatly the limits of the volcanic 

 rocks have been reduced by denudation. There can be little doubt 

 that at least the finer tuffs extended westwards as far as a line 

 drawn from Tremadoc to Llanegrin — that is, some fifteen miles or 

 more beyond the cliffs of Aran Mawddwy, thus stretching across 

 much of the site of what is now the great Harlech anticline. 



This compact, well-defined volcanic area, in spite of the faults 

 which traverse it and the disturbed positions into which its rocks 

 have been thrown, is, in many respects, one of the simplest and 

 most easily studied among the Palaeozoic formations of this country. 

 Its main features have been delineated on the maps of the Geological 

 Survey and have been described in the Memoir. But these publi- 

 cations cannot be regarded as more than a first broad, though mas- 

 terly, outline of the whole subject. There is an ample field for 

 further and more minute research wherein, with the larger and better 

 Ordnance maps now available, and with the advantage of the nume- 

 rous modern petrographical aids, a more exhaustive account may be 

 given of the district. The whole volcanic succession from base to 

 summit is laid bare in innumerable magnificent natural sections 

 along ranges of hills for a distance of some forty miles, and a careful 

 study and re-mapping of it could not fail to add greatly to our know- 

 ledge of the early history of volcanic action *. 



According to the observations of the Geological Survey, the vol- 

 canic rocks of Merionethshire naturally arrange themselves in three 

 great bands, each of which is described as tolerably persistent 

 throughout the whole district : — 1st. A lower series of ashes and 

 conglomerates, sometimes 3300 feet thick (Aran Mawddwy) ; 2nd. 

 A middle group of " felstones " and " porphyries," consisting partly 

 of true contemporaneous lava-streams and partly of intrusive sheets, 

 and reaching a thickness of 1500 feet ; 3rd. An upper series of frag- 

 mental deposits like that beneath, the extreme thickness of which is 

 800 feet (Arenig). A re-mapping of the ground en the six-inch 

 maps would, no doubt, show many local departures from this general 

 scheme. 



* The excellent conjoint papers of Professor Cole, Mr. Jennings, and Mr. 

 Holland are, I hope, an earnest of what may be expected from them in their 

 examination of this deeply interesting region 



