82 Physical Geography. 



thickness in the same direction, till, in the Bala country 

 and further north, they are represented only by a few 

 insignificant beds of ashy strata, a character of which 

 the Bala limestone itself sometimes feebly partakes. 

 The idea is, that the prevalent westerly winds had a 

 tendency during eruptions to blow the volcanic dust 

 and lapilli eastward, and that these materials fell thick- 

 est near the vents and at middle distances, and gradu- 

 ally decreased in quantity the further east they were 

 carried. 



To those unaccustomed to technical geological argu- 

 ments a word of warning remains. Let no reader 

 suppose that in Wales he will now find clear traces of 

 these old volcanic cones and craters in their pristine 

 form, such, for example, as the extinct craters of 

 Auvergne and the Eifel. Semi-circular hollows sur- 

 rounded by igneous rocks like those of Cwm Idwal anf* 

 Cwm Llafar he will find plentiful enough, and these, 

 in old guide-books and other popular literary produc- 

 tions, have sometimes been described as craters. So far 

 from that being the case, such cirques or corries are 

 ancient valleys of erosion, the rocks of which have been 

 exposed to the weather perhaps ever since Upper Silurian 

 times, and have been subsequently modified by glaciers, 

 during the last Glacial Epoch, in days, comparatively 

 speaking, not far removed from our own. The truth about 

 these ancient volcanoes is, that long after they became 

 extinct the whole Lower Silurian area was disturbed 

 and thrown into anticlinal and synclinal curves, which 

 suffered denudations before the beginning of the de- 

 position of the Upper Silurian rocks, and the positions 

 in which the lavas and ashes now stand will approxi- 

 mately be best understood, if we suppose Etna by 

 similar disturbances to be half turned on its side, 



