144 



Geological Society, 



f Gelli-Grin Calcareous Ash, a 



100 feet, with Gelli-Griu ^ 



Moel-Fryn, Bryn-Pig, & =« 



Caerhafotty Limestones. -S 



Pont-y-Ceunant Ash, maxi- | 



mum 25 feet. g> 



AUt-ddw Mudstones, with | 



thin limestones, 1300 feet. -S 



Pronderew Ash, 12 feet. J 



Glyn-Gower Sandstones, 'S, 



with thin limestones, S 



1^ 1100 feet. I 



f Nant-hir Shales and Derfel -g 

 Limestone. '>^ 



f (a) Chasmops and 

 Orthis (Nico- 

 lellaj ar.tonise 

 sub-fauna. 



(b) Asaplin.s-powlsi 

 and Heterorthis 

 alteniata sub- 

 fauna. 



Dlcra)iograptus 

 Shales. 



1 Zone of Dicrano- 

 \ graptus clingani. 



Zone of Cliinacograp- 

 tus peltifer or ^e- 

 magraptus gracilis. 



The so-called ' Bala Limestone ' is merely one of a series of lime- 

 stone lenticles occurring within the Calcare(jus Ash at different 

 horizons. The base of the Ashgillian appears to be calcareous 

 everywhere west of a definite north-and-south line. There has 

 been some confusion between the Rhiwlas Limestone and the lime- 

 stones in the Calcareous Ash ; but at Bryn Pig, where both are seen 

 together in vertical section, the lithological and faunal differences 

 are manifest. 



The detailed mapping of the beds, as now classified, has brought 

 out the structure of the country more completely than was hitherto 

 possible, and a modification of views previously held with regard 

 to the Bala Fault seems to be necessary. It apj^ears to be 

 one of a series of compressional faults affecting the whole of the 

 country south-east of Bala Lake. 



The initiating structural factor was probably compression of 

 the rocks as a whole against the Harlech Dome, controlled by 

 the resistance offered by the Ordovician volcanic mass to the 

 compressional force, which affects the detail of the structure of 

 the whole country lying east and south-east of it. The cotmtry 

 was first folded, and then affected by thrust-movements. There 

 are six main structural lines of displacement: — (1) The Llj'n- 

 Tegid line ; (2) the Bala-Lake line ; (3) the Llangower line ; 

 (4) the Cefn-ddwy Graig line ; (5) the Moel-Fryn line ; and 

 (6) the Fridd-defaid line. 



Combined with these major displacements, there has been 

 much differential _ minor thrusting (tears), which is most con- 

 spicuous above the Llangower thrust. The effect of this thi-usting 

 diminishes steadily from west to east, and in the Hirnant Valley 

 the beds are being compressed without any faulting. 



Comparison is made between the succession here seen and that of 

 other areas in Wales, Shropshire, the Lake District, and the South 

 of Scotland, and the faunal features are noted and tabulated. An 

 interesting feature comes to light : namely, the approximation 

 of the Derfel-Limestone fauna to that of the Stinehar Limestone, 

 rather than to that of anv Welsh beds hitherto described. 



