294 



AREN1G VOLCANOES IN NORTH WALES. 



localities, is associated with volcanic rocks, and is known to result 

 from their decomposition. 



Arenig Volcanoes. It was during the succeeding Arenig period 

 that the volcanoes in the Welsh district attained their greatest acti- 

 vity. There are no remaining traces of the throats up which the 

 igneous matter was ejected, for these outbursts seem analogous to recent 

 submarine eruptions among the Canaries, which rose from the floor of 

 a moderately deep sea, and in a few years were so completely worn away 

 that no definite indications of their position can now be ascertained. 



In Cader Idris the Arenig rocks include a jointed porphyry, some- 

 what hornblendic, and therefore possibly andesitic, which is 1700 

 feet thick, and extends northward continuously to beyond Aran Mowd- 

 dwy. Upon these lava streams are 300 feet of blue slates, formed of 

 muds which may have been to a large extent derived from the decom- 

 position of volcanic materials ; and higher up are i oo feet of por- 

 phyritic felspathic ashes, and finally 500 feet of a greenstone, which 

 is full of air cavities, and therefore was poured out under moderate 

 pressure of water or air. Many intrusive sheets of greenstone also occur. 

 Lower down the mountain, the newer beds also alternate with fel- 

 spathic ashes and lavas, which are rudely columnar, and are about 

 1500 feet thick; but Sir Andrew Kamsay is doubtful whether the 

 lavas are really contemporaneous, and out of the 3600 feet of igneous 

 materials interstratified with the slates, believes that only 400 feet of 

 ash-beds can with certainty be regarded as derived from craters which 

 were active during the Arenig period. 



The Arans, however, have the same general structure as Cader 

 Idris, and there is here less room for doubt as to the contemporaneous 

 character of the interstratified igneous rocks, because the lavas have 

 baked the slates over which they flowed, giving them the texture of 

 porcelain, while the slates which rest upon the lavas are unaltered. 

 The volcanic ashes are frequently vesicular, and sometimes water 

 worn, and thin away rapidly on the north side of the Aran chain, as 

 though that were furthest away from the volcanic crater. 



Evidence of this thinning out of the ashes and lava is best exhi- 

 bited in a table, constructed from Sir Andrew Ramsay's data: 



from which we learn that though the contemporary ashes and lavas 

 extend from Cader Idris to the east of Festiniog, and though the ash 



