208 DE. WILLS AXD ME. SMITH OX THE LOWEE [vol. Ixxviil, 



(c) Intrusions in the Lower Ludlow Series. 



An olivine-dolerite (tlioleite) occurs as small dykes in a few 

 places in the Lower Ludlow rocks. Mr. Lake mentions one near 

 Pen-j-vivod. Another outcrop is in the Afon-Eo, about 200 yards 

 from its confluence with the Dee ; but there is no proof that 

 the two form parts of the same dyke. Although the rocks have 

 not been found in situ, there is evidence of a third occurrence on 

 Craig-y-Khos, a hill half a mile north of G-lyn-Dyfi'dw}'. 



(4) ? Upper Ludlow Series. 



Dinas-Bran G-roup. — The rocks forming the conspicuous hill, 

 crowned with the remains of the ancient fastness of Castell-Dinas- 

 Bran, have long been regarded as the highest Silurian in the area. 

 They are uneleaved, thinly-bedded, slightly calcareous, sandy shales, 

 usuallv micaceous, and occasionallv fossiliferous. 



Mr.^ Lake (Q. J. a. S. vol. li, 1895, p. 20) gives a list of fossils 

 from here, several of which have been also collected during the 

 present survey. The following new records may be added : — • 



Chonetes cf. sfriateJla Dalman. 

 Orthis cf. lunata J. de C. Sowerby. 

 Orthis cf. orhicidaris J.de C.Sowerbv. 



Orthonota cf. amijgdalina J. de C. 



Sowerby. 

 Pleurotomaria sp. 

 Ortlioceras liidense J. de C. Sowerbv. 



Dayia na vie id a is the commonest fossil here, and has also been 

 found at various other localities nearer Llangollen. Mr. Lake 

 suo-gested that the fauna indicated an L^pper Ludlow age. Since 

 then Miss Gr. L. Elles & Miss I. L. Slater ^ have described in detail 

 the Ludlow Series at Ludlow, giving an extensive list of fossils. 

 According to these authors, Dayia navicuJa is not found in the 

 Upper Ludlow. Its abundance in the Dinas-Bran Grroup suggests 

 a correlation of that with the Mocktree or Dayia Shales ; but, as 

 these fall in the zone of jSL. leintwardinensis, which at Llangollen 

 is known to underlie the Dinas-Bran Beds, and as, with this 

 exception, the fossils are, generally speaking, common to both 

 Lower and Tapper Series, we are inclined to agree with Mr. Lake 

 in assigning the group to the Tapper Ludlow. 



IV. Steucttjees. 



In the succeeding portion of this paper, we attempt to describe 

 the structures that have resulted, in the main, from mountain- 

 building movements in Devonian times, although in part from a 

 continuation of the cycle of movements into later ages. But the 

 later movements have merely modified, and not obliterated, the 

 major tectonic featui-es that were rough-hewn by the Caledonian 

 folding. 



1 ' The Highest Silurian Eocks of the Ludlow District ' Q. J. G. S. toI. Isii 

 (1906) pp. 195-222. 



