part 3] lower palaeozoic of arthog-dolgelley/. 309 



the outcrops. The synclinal structure is well displayed in the 

 -curves of the ridges above Capel Horeb, as viewed from Barmouth 

 Bridge. 



(ii) The cleavage. — The effects of cleavage are well marked 

 throughout the district, all the softer beds having been affected : 

 sometimes also some of the harder beds, such as the rhyolitic 

 ashes. On the other hand, the cleavage-stresses have failed to get 

 any grip on the rapid alternations of arenaceous and argillaceous 

 material that build up the Basement Series. 



Although all the argillaceous rocks are now in the condition of 

 .slates, the cleavage has not usually been sufficient entirely to 

 destroy the fossils. The cleavage-strike usually coincides very 

 nearly with the bedding-strike, but is less susceptible to variations 

 m direction. Evidently, therefore, the cleavage was not established 

 until after the minor folding had taken place. The cleavage-dip 

 usually is steeply inclined to the south, as compared with an 

 .average bedding-dip of only -i0° southwards. Other things being 

 -equal, fossils are therefore more readily obtainable where the beds 

 are more highly inclined, since cleavage and bedding then more 

 nearby coincide. As examples may be cited the locality for the 

 Upper Dictyonema Band in the Gwynant Valley and the localities 

 for Didymograptus bifidus near Llynau Crogenen. 



Although the intrusive rocks are earlier than the cleavage, they 

 rarely show cleavage effects, even along the edges of the smaller 

 intrusions ; exceptionalhy, however, examples of diabase have been 

 found so sheared as to simulate an ash or a felspathic grit. The 

 more highly metamorphosed examples of contact-altered rocks 

 .likewise display little or no trace of cleavage. 



(iii) The faulting. — The faults may be classified as follows: — 



,(ci) Great faults nearly parallel to the strike. 



(i) Normal faults with a downthrow to the south. Examples : — The 



Derwas, Dolg-elley, and Mynydd-Gader North Faults, 

 (ii) Reversed faults with an upthrow to the south. The Ceunant- 

 Gwernan Fault. 



<(b) Small faults transverse to the strike. 

 (i) Dip-faults, very numerous. 



(ii) North-west faults, examples on Bryn Brith and Gelli-llwyd. 

 (iii) North- and- south faults, examples- at Llyn Wylfa and the Arthog- 



Valley, 

 (iv) Fractures at those points where the strike undergoes a sudden 

 change, as near Pant-y-Piod. 



All the varieties of transverse faults are quite small, and so 

 their effects can hardly be shown on the map. In the field they 

 frequently give rise to small notches where they cross the harder 

 bands. 



The greater faults that keep more nearly parallel to the strike 

 deserve more detailed mention. Their exact relationships one to 

 the other, to the folding, and to the intrusive rocks, are rather 



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