THE VOLCANIC HISTORY OF NORTH WALES. 293 



rocks at Bangor, and part of those at Llyn Padarn, are ancient lava- 

 flows, which in strictness should be termed rhyolites. This felsite, 

 under the microscope, shows a ground mass containing crystalline grains 

 of quartz, with orthoclase and plagioclase. The rock constantly ex- 

 hibits the streaky structure characteristic of acidic lavas, while to the 

 north-east of Llyn Padarn the felsite is associated with agglomerate. 

 These masses are referred by Professor Bonney to the Pebidian group 

 of Dr. Hicks. 



On the eastern side of the Malvern Hills, near the Herefordshire 

 Beacon, there is a small area of compact felsitic rocks of uncertain 

 age, which Professor Bonney regards as probably Pebidian. 1 



Pre- Cambrian Volcano of Charnwood Forest. In Charnwood 

 Forest Professor Bonney and the Rev. E. Hill have described rocks 

 probably of pre-Cambrian age, which consist of slates alternating 

 with thick masses of rhyolitic agglomerate, less glassy than those of 

 the Wrekin, and alternating with beds which appear to be composed 

 of volcaiiic materials, sorted by water and somewhat triturated. 



Cambrian Volcanoes. 



Cambrian Volcanoes of North. Wales. The great importance 

 of volcanic rocks in the history of the Cambrian Formations of North 

 Wales may be best appreciated by examining a geological map, which 

 represents on a sufficient scale the country from the south of Bar- 

 mouth through Cader Idris, the Arans, and the Areing Mountains 

 into Caernarvonshire. On the Geological Survey map broad strips of 

 reddish colours indicate the areas now occupied by beds of volcanic 

 ashes, felspathic lavas, and quartz porphyries, which were poured 

 out from sub-aerial and sub-marine volcanoes during the whole of the 

 Cambrian ages ; and in the Malvern district the Holly bush sand- 

 stone contains dolerites 1 which are interbedded, and at least as old 

 as the Lingula Flags of North Wales. The lava occurs as lenticular 

 masses, which now stand up in rounded bosses, because they have 

 decomposed less rapidly than the muddy sediments in which they 

 were covered up. 



Volcanic Rocks in the Lingula Flags. The Lingula Flags of 

 North Wales in their higher beds so resemble volcanic ashes that 

 between Capel Arthog and Penmaen, west of Dolgelly, there appears 

 to be no doubt about their having originated in volcanic action. 

 But, otherwise, the igneous rocks of the Lingula beds are for the 

 most part dykes and intrusive masses of dolerite, though some 

 masses of felsite occur. Many of these dykes run in the line of 

 strike, but they are never interstratified, and there is no evidence of 

 their geological age. 



Volcanoes of the Tremadoc Slates. Volcanoes were probably 

 active during the succeeding Trernadoc period, for near the top of 

 that series the slates contain pisolitic iron ore, such as, in other 



1 Holl : Q. J. G. S., vol. xxi. p. 72. 



