76 Igneous Rocks. 



melted matter, the eruption of which to the surface 

 through volcanic rents, produced the lava-flows and 

 ashes already mentioned. The ashy beds are sometimes 

 coarse and tufaceous, but were also often formed of 

 fine volcanic dust, which being now consolidated into 

 hard felspathic rocks, are at first sight somewhat diffi- 

 cult to distinguish from the associated lavas. Practice, 

 however, renders it comparatively easy, and in distin- 

 guishing the difference, the observer is aided by the 

 circumstance, that underneath each lava current the 

 slates, once beds of mud, are apt to be baked and por- 

 cellanised at the point of junction with the originally 

 hot lavas, which having in the meanwhile cooled, the 

 slaty beds that rest on them are in that respect un- 

 altered. 



The second series of eruptions may be traced 

 as follows. Near Bala, not far below the limestone, 

 there are a few thin bands of volcanic ashes. These, 

 as we go northward to the rivers Machno, Lledr, and 

 Conwy, gradually thicken, and by-and-by get mingled 

 in that slaty area with numerous thin and thick bands 

 of felspathic lavas, the importance of which as large 

 masses, culminates in Snowdon and the surrounding 

 area, going northward by Grlyder-fawr, Grlyder-fach, 

 Carnedd Dafydd, and Carnedd Llewelyn, and so on to 

 Conway. South of Snowdon the same kinds of lavas 

 and ashes are seen in force on the sides of Moel Hebog, 

 and the great mass of Llwyd-mawr near Dolbenmaen. 



Other large bosses of intrusive rocks, mostly fel- 

 spathic, occur on Y-Foel-fras, between Snowdon and 

 Conway, another between Llanllyfni and Bethesda, a 

 third near the eastern shore of Menai Straits, and 

 many more including the beautiful mountains of Yr 

 Eifl, or The Eivals, in the north horn of Cardigan Bay, 



