part 3] lower paljeozoic of arthog-dolgelley. 285- 



are usually obscured by slipped material. Nevertheless, the actual 

 base of the intrusion is visible at various points, especially along. 

 the slopes of Pared-y-Cefn Hir, and it is seen to dip in the same 

 direction, and at nearby the same angles as the adjacent stratified 

 beds. The top of the intrusion is not well displayed along the 

 central outcrop ; but, where it is repeated, below Tyrau-mawr, the- 

 relations are clear enough, and the upper boundary of the intrusion 

 dips with the superincumhent strata. Moreover, the rock almost 

 invariably possesses a columnar jointing, and the columns, wherever- 

 found, show a uniform ' dip ' ; they are, therefore, presumably 

 arranged at right angles to the top and bottom of the intrusion- 

 Now, the direction of the columnar jointing is exactly such as would 

 be expected in a sill dipping approximately with the adjacent 

 sedimentary rocks. 



The evidence from the distribution of the outcrops, the nature 

 of the junctions, and the character of the columnar jointing, alL 

 combine, therefore, to show that the intrusion is everywhere to be 

 regarded as a sill. This clear proof of the sill-like habit is the 

 more interesting when account is taken of the steady and gradual 

 transgression of the intrusion into higher beds when followed 

 from north-east to south-west. 



When we first began work in the district it seemed possible- 

 that the appearance of transgression was really due to faulting 

 along the margin of the intrusion. This view gained support from 

 the fact that the margins of the main sill are overlooked in each 

 case by scarps, the one on the south side being particularly pro- 

 minent. These scarps are formed by the various sedimentary and 

 igneous rocks that are in contact with the granophyre, the scarp- 

 itself being almost independent of the kind of rock of which it is 

 composed. This feature is especially striking on Gelli-llwyd, 

 where the well-marked scarp is composed, in different parts of its 

 course, of diabase, of various rhyolitic rocks, and of Bryn Brith 

 Beds. The course of the scarp is quite independent of the run of 

 these various beds which strike, indeed, diagonally across the 

 feature. It seemed possible, therefore, that this scarp was nothing 

 but a fault-feature. Further examination, however, revealed the 

 presence of the peculiar basic rocks characteristic of the granophyre 

 margin, while elsewhere actual chilled margins of granophyre 

 were discovered. Obviously, therefore, the junctions of the grano- 

 phyre with the sedimentary rocks were normal and not faulted, 

 and it follows that the appearance of transgression displayed on 

 the map (PI. XX) is due to an actual transgression, and not to- 

 faulting. 



The gradual transgression and the sill-like habit of the intrusion 

 were noticed by Ramsay ('Geology of North Wales ' Mem. Geol. 

 Surv. vol. iii, 1st ed. 1S6G, p. 25), who remarked that the intrusion 

 took place 



' before those later disturbances occurred that threw both stratified 



and igneous rocks into their present curved and inclined position ' 



