144 



Geological Society. 



f Gelli-Grin Calcareous Ash, g 



100 feet, with Gelli-Grir. 3 



Moel-Fryn, Bryn-Pig, & =*^ 



Caerhafotty LimestoneB. 5 

 Pont-y-Ceunant Ash, maxi- | 



mum 25 feet. g» j 



Allt-ddw Mudstones, with | | (b) AsapJ 



f (a) Chasrnops and 

 Orthis (Nico- 

 lella) ac.tonise 

 sub-fauna. 



ts-pow\ 



and Heterorth 

 alternata sub- 

 fauna. 



-1 



Zone of Dicrano- 

 graptus clingani. 



thin limestones, 1300 feet, -g 

 Fronderew Ash, 12 feet. _§ 



Glyn-Gower Sandstones, U 



with thin limestones, S 



1100 feet. I 



Nant-hir Shales and Derfel "I Dicmnograptus 



Limestone. '^ Shales. 



Zone of Climacograp- 

 tus peltifer or Ne- 

 magraptus gracilis. 



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

 stone lenticles occurring within the Calcareous Ash at different 

 horizons. The base of the Ashgillian ajjpears 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 

 too-ether 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 appears to be 

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

 country south-east of Bala Lake. 



The initiating structm-al 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 country 

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

 are six main structural lines of displacement: — (1) The Llyn- 

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

 (4)"' the Cefn-ddwy Graig Hne ; (o) the Moel-Fryn line; and 

 (6) the Fridd-defald line. 



Combined with these major displacements, there has been 

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

 spicuous above the Llangower thi-ust. The effect of this thrusting 

 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 Stinchar Limestone, 

 rather than to that of any Welsh beds hitherto described. 



