162 DB-. G. L. ELLES OX THE BALA COUNTRY : [vol. Ixxviii, 



while the Bryniau-goleu tear shifts the same bed when practically 

 vertical for a distance of half a mile ; the Beudy-Graienyn tear is 

 the last of the big tears to bring about a noticeable horizontal 

 movement of the beds. That all have probably been shifted to 

 some extent is, nevertheless, suggested by the outcrop of the 

 Frondderw Ash between the two main outcrops north-east of 

 Cornelau farm; this can only be interpreted as implying a wholesale 

 shortening of the line by differential movement north-westwards. 



The tear-faults associated with the Moel-fryn displacement are 

 less important, although there is an interesting little group of them 

 on Bryn-cut, north of the Hirnant valley, where the outcro^js of 

 the Grelli-grin Limestone and the Pont-y-Ceunant Ash are con- 

 spicuously shifted. 



These tear-faults are also distinguished by definite topographical 

 features, since the}^ are nearly always marked by a gash limited by 

 wall-like outcrops of the rock on each side, the width of the gash 

 varying with the importance of the tear; they also are almost 

 invariably accompanied by more or less horizontal slickensiding 

 and by intense quartz mineralization. 



VI. Paljeoxtologt. 



There are certain features of interest in the faunas found at 

 different horizons in the Bala district. In the first place, the 

 fauna of the Derfel Limestone is a remarkable one, for, from 

 the list given on p. 145, it will be seen that in its general character 

 the fauna is more closely related to that of the Scottish rather than 

 to that of the Welsh Llandilian rocks. Brachiopods are far more 

 conspicuous than is usual in the Welsh Llandilian, and there seems 

 to be no trace whatever of the trilobites which are of common 

 occurrence in the Welsh beds of that age, such as Trinucleus 

 Jimhriatiis^ Aiwpyx nudus, Ogygia huclii, Barrandia radians, 

 S. cordai, Calymene duplicata, and other forms, which even 

 as near as Builth (Gwern-y-fed-fach) occur associated with the 

 same graptolites as those that are found in Nant- Derfel. 



Anyone who has studied the faunas of the Lower Ordovician 

 rocks of Scotland must have realized that they are essentially the 

 forerunners of the Ashgillian fauna, and some of the forms found 

 in the Nant-Derf el gorge appear to be identical with some occurring 

 in the Ashgillian itself. The occurrence, therefore, of this type 

 of fauna at this horizon in this area is of great palseontological 

 significance. 



As respects the faunas of the Caradocian rocks, while the 

 grouping here adopted is necessarily based mainly upon the 

 assemblages found in the Bala country, the assemblages found in 

 other parts of England and Wales have also been taken into 

 consideration. 



With regard to the general aspect of the Caradocian fauna, 

 it may be noted that it is essentially a trilobite-brachiopod fauna. 

 It is very generall}'" rich in brachiopods of large size, many of 



