102 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 2, 



where the beds join on to undisturbed masses lying bevond the line 

 AB. 



If this operation is continued until the beds give way, their fracture 

 must be expected where they are most bent, that is, on each side 

 somewhere about the points d d: and as the pressure is upwards, two 

 anticlinal ridges will be formed over those points near the boundaries 

 of the area and parallel to it. Their importance will be much in- 

 creased if a continuance of the pressure forces part of the fluid up- 

 ward through the rent thus formed. On each side of this elevated 

 ridge we should expect at least one synclinal axis resulting mechani- 

 cally from the raising upwards of a portion of the surface. And 

 between the anticlinal ridges and the boundaries of the area, the 

 beds would be much crushed together and broken, but not elevated. 

 Thus in each great area of elevation the principal features of distur- 

 bance should be the raising up of the beds in an arch or anticlinal axis 

 along the central line, an important anticlinal axis near each of the 

 boundary lines, with great confusion between that axis and the 

 boundary, and a synclinal axis on each side between the centre and 

 the anticlinal axes : besides these, there may be many other minor 

 disturbances parallel to and depending upon those principal ones. 

 The outline of the area thus described is shown in fig. 23. 



C. The central arch or anticlinal axis. 

 D D. The great lateral anticlinal axes. 

 S S. The principal synclinal axes. 

 B B . Disturbed districts near the boundaries of the area. 



If the preceding reasoning is correct, we should find this combi- 

 nation of features common ; the proportions varying in every case, 

 and the whole modified by local causes ; and every district presenting 

 all these characters must be considered a complete area of elevation. 

 Of Mr. Hopkins's sections across the Wealden elevation*, Nos. 18, 

 20, 22 and 23 correspond more or less closely to the ideal section 

 given above. The elevation of Carnarvonshire agrees exactly with 

 this section, and in fact it suggested the view here taken : see fig. 17. 

 Its principal features are the central axis of Rhaiadr Cwm, the syn- 

 clinal axis between the centre and Snowdon, and the great anticlinal 

 axis of the Snowdon chain, on the outside of which the beds are low 

 but in great confusion. The Snowdon anticlinal has been raised to 

 a disproportionate height by the igneous matter forced up through it. 

 On the Merionethshire side of the area the features are less regular, 

 but some of them may still be seen. 



It follows from the preceding reasoning that the elevating move- 

 ment of which the axis passes Rhaiadr Cwm, and which gave their 



* Hopkins on the Wealden District, Transactions of the Geological Society, 

 2nd Series, vol. vii. pp. 46 and 47. 



