114 ME. H. IAPWOETH OX THE [Feb. IC;00, 



(2) Overlap. — 'Overlap occurs in a conformable series when 

 each succeeding bed stretches beyond the limits of that below it in 

 one or more directions, so as to have a wider extension, and to 



conceal the edges of the lower beds Overlap is indeed a 



necessary consequence of the underlying surface of denudation 

 being an inclined plane instead of a horizontal one, consequently 

 any slope in an area of subsidence will give rise to the phenomena 

 of overlap.' x In brief, overlap involves : — (i) A sloping surface on 

 the older area of denudation, (ii) The natural thinning-out of the 

 individual beds of the overlying series before they are overlapped, 

 (iii) Subsidence in the area of deposition, (iv) Unconf or m ability 

 between the two groups : this follows of necessity, (v) An ex- 

 tension of each succeeding bed wider than that of the underlying 

 one. 



Let us examine the Caban Group and see how these conditions 

 apply. We have already noticed that the conglomerates thin out 

 eastward and westward, and that they are at their thickest in the 

 central area. The reason of this is simple, in view of the fact that 

 the greatest amount of denudation of the Gwastaden Group has 

 taken place in the central portion of the western area, where the 

 conglomerates rest nearly on the Cerig Gwynion Grits ; while at 

 the eastern and western limits they repose on beds between 100 

 and 200 feet higher up in the group. That is to say, we have 

 phenomena satisfying condition (ii). The wider extension of the 

 beds has been demonstrated ; for the Monograptus-Sedgwickii Grits 

 extend farther eastward than the conglomerates ; they reach, in fact, 

 beyond the point where the conglomerates disappear. The overlying 

 Gafallt Shales again, would, if produced under the river-alluvium, 

 extend to Glan Elan, about 500 yards still farther east, while the 

 Rhayader Pale Shales may be carried on for at least 4 or 5 miles 

 beyond the point at which the whole of the Caban Group has 

 vanished. Of condition (v) we have already had proof. 



So much, then, for the general facts which go to prove overlap. 

 If, however, further proof be required in detail, it may be obtained 

 in the high ground between Pen-y-rhiw, Coppa, and Dol-Ifor Farms. 

 If this area be carefully mapped on a large scale, it will be found that 

 each successive ridge or hollow, representing some division of the 

 Gafallt Group, will gradually approach the edge of its under- 

 lying bed, pass over it, and be eventually covered up by the next 

 higher division. The base of the Gafallt Group (Bb^ may be 

 easily traced over the Upper Conglomerates (Ba 3 ). This line of 

 separation takes the form of a distinct groove sweeping over the 

 hill in a sharp curve, which no fault could possibly produce. It 

 approaches the conglomerates by degrees, and, at the boundary- 

 hedge, apparently passes over their edges, ultimately coming to 

 rest on the underlying Gwastaden rocks. Another ridge, at the 

 foot of the wood above Copra Farm, may be traced without 



1 A. J. Jukes-Browne, ' Students 1 Handbook of Physical Geology' 1884, 

 p. 384. 



