part 3] LOWER PAI/^OZOIO of abthog-dolgelley. 307 



We ma}^ consider next the relationships of the intrusions to the 

 tilting and folding. It has already been shown (p. 281) that the 

 diabase-intrusions, while acting in a general way as sills, yet tend 

 to rest upon the edges of, and to conceal, the stratified rocks in such 

 a way that the sills must obviously transgress southwards on to 

 stratigraphically higher beds. This is the case with the Cae-Einion 

 Gelli-llwyd sill. Similar relations have been shown to prevail in 

 the case of the great diabase-sill of Mynydcl-}'- Grader, also of the 

 various diabase-masses nearer Dolgelley, which in fact are simply 

 continuations of the Gelli-llwyd sill. 



As regards the acid rocks, their intrusion took place later than 

 that of the basic rocks, as is clear from the field relations (p. 282); 

 but the interval between the two series of intrusions was probably 

 not great. Now, in the case of the chief acid intrusion, the 

 northward transgression onto lower stratigraphical levels is still 

 more marked than in the case of the basic rocks. These facts lead 

 to the conclusion that both basic and acid intrusions took place 

 during a period when tilting movements had already established 

 their sway. It appears as if the magmas rose to well-defined 

 hydrostatic levels. In the case of the earlier diabase-intrusions the 

 tilting had only just started, so that the hydrostatic level was 

 nearly, but not quite, parallel to the bedding of the stratified 

 rocks. By the time the later granophyre-magma arose tilting 

 had proceeded still further, Avith the result that the divergence 

 between the hydrostatic level and the dip of the strata Avas more 

 pronounced ; hence the more obvious transgression of the acid 

 intrusions. The tilting had not yet, however, proceeded so far as 

 to destroy the general sill-like nature of the intrusion. It was, 

 in fact, merely an early forerunner of, and acting in the same 

 direction as, the great folding movements which, later, threw both 

 sedimentary and igneous rocks into their present positions. 



Further evidence that the tilting and folding movements had 

 already begun before the igneous rocks were intruded is obtained in 

 the Arthog Valley. There all the strata are affected by a shalloAV 

 anticline which runs transverse to the normal strike. A diabase- 

 sill on approaching the anticlinal axis transgresses through the 

 Basement Series and down on to the Tremadoc Slates, but resumes 

 its former level so soon as it escapes from the fold. This suggests 

 that folding had begun before intrusion took place, and that the 

 hydrostatic level was independent of the fold as then established. 

 That folding Avas continued subsequently to intrusion is obvious 

 from the manner in Which outcrops are affected. 



If the tilting movements had commenced thus early, so that 

 the igneous intrusions overlap, as it Avere, on to loAver horizons in 

 the north, it follows that a similar though less marked overlay) 

 should exist between the various divisions of the Ordovician sedi- 

 mentary rocks themselves. Our knowledge of the detailed strati- 

 graphy of the Ordovician rocks of North Wales is still incomplete ; 

 but there is some evidence as to the existence of such an overlap. 

 Its presence in the district between Arenig and Tremadoc has. 



Q. J.G. S. No. 303. t 



