VOLCANIC ROCKS OF THE LAKE DISTRICT. 295 



extends from six miles south of Barmouth to six miles west of Bala 

 Lake, all the beds die away and disappear at a little beyond Tremadoc 

 on the north, and towards Llanegryn on the south. The lower ashes 

 have their greatest thickness in Aran Mowddwy, where the lava also 

 is thickest. The lower ash disappears in the Arenig. The lavas have 

 thinned to an insignificant amount in Arenig, while the upper ash 

 there attains its greatest thickness. This deposit formed a lenticular 

 mass 23 miles long, thinning away on the south near Penmaen, and 

 on the north at Cwmorthim. The volcanic centres of this region are 

 placed by Kamsay to the eastward of a line drawn from Tremadoc to 

 Llangryn, and he suspects that the felspathic masses of Tyddyn-Rhiw 

 and Gelli-llwyd-fawr near Dolgelly, and of Y-Foel-ddu near Aran 

 Mowddwy, and part of the Arenig, are probably remains of the necks 

 of volcanoes, because the lower ashes and felstones attain their 

 greatest development near Dolgelly and Aran Mowddwy, and the 

 upper ashes attain their greatest thickness near Arenig. 



The greenstones do not appear to have at any time reached the 

 surface of the country, and therefore it is difficult, if not impossible, 

 to fix their age ; even the great mass of Rhobell Fawr, which is seven 

 miles long and three miles broad, appears to have been an intrusive 

 overflow between planes of bedding, like the masses termed laccolites. 



In the lake country contemporary volcanoes poured out great 

 thicknesses of felspathic lava and ashes, which are interstratified in 

 the formation known as Skiddaw slate. 



Llandeilo Volcanoes. In the succeeding period, known as the 

 Llandeilo Flags or Lower Bala Rocks, volcanic action still continued 

 with great vigour. In the group of green slates and porphyries of the 

 Lake district, the volcanic rocks make as prominent a feature as in 

 strata of the same age in Wales ; but according to Professor Bonney 

 these are rather to be referred to the andesites, since they correspond 

 to them in chemical composition. 



The Volcanic Rocks of the Borrowdale Series. The greenslates 

 and porphyries of Sedgwick, termed the Borrowdale series by Pro- 

 fessor Nicholson, cover a large area of the central part of the Lake 

 district, extending 25 miles E.N.E. along the strike, and 13 miles 

 S.S.E. in the direction of the dip. These rocks are estimated at 5000 

 to 6000 feet thick, and a large part of the thickness consists of asli 

 beds and lava streams. In Borrowdale, the Skiddaw slates are over- 

 lain by dark -green compact lava, which is sometimes rudely columnar. 

 This is succeeded by felspathic ash and agglomerate, with many 

 minor beds of lava, the highest bands being amygdaloidal ashes, in 

 which the cavities are commonly filled with quartz. These rocks are 

 well seen rising through Cat Bells, Barrowside, Maiden Moor, and 

 Narrow Moor. A similar sequence is seen in the valley of Gates 

 Garth Beck, which flows into the head of Buttermere. In many 

 places the ashes and breccias are cleaved, and form fine-grained slates, 

 sometimes green, sometimes purple, as may be seen in the quarries at 

 .Dale Head and Honister Crag. Sometimes a subordinate bed of fine 

 purple felstone occurs with large greenish crystals of felspar. Similar 



