part 2] 



ITS STRTJCTTJEE AIS^D ROCK-SUCCESSION. 



149 



from the limestone outcrop, which in places is practically vertical. 

 The Grelli-grin Limestone is here particularly pure, and has yielded 

 only a few fossils : — 



Orthis (NicolellaJ actonias (Sowerhj). Christiania tenuicincta (M'Coy). 



Ortliis (Dahnanella) elegantula var. Hyattella 'portlockiana (Davidson). 



Orthis fPlatystrophiaJ biforata Lingula ovata Salter. 



(Schlotheim). Monticuliporids. 

 StrojyJiomena fBafinesqniiiaJ ex- 



pansa var. 



The base of the limestone is a massive, blue-gre}?-, crystalline 

 rock; but it becomes more concretionaiy towards the top, and is 

 immediately overlain by the pale-gre}^ pasty Rhiwlas Mudstone, 

 which close to Plas-Khiwaedog contains : — 



Encrinurios sexcostatus Salter. 



Cyhele verrucosa (Dalman). 



Illsenus davisi Salter. 



Cheiriirus sp. 



Liclias sp. 



Stmtrocephalus Tmirchisoni Barrande. 



Trinucleus sp. 



Orthis (DalmanellaJ elegantula var. 



drummucTcensis Reed ? 

 Orthis fHehertellaJ crispa (M'Coy). 

 Cystid-plates. 

 Primitia. 



Most of the beds seen on Bryn-cut are also visible on the Lake- 

 Yyrnwy road, with the exception of the Gelli-grin Limestone, 

 which has been faulted out ; on the other side of the valley the 

 rocks sweep up and round to the classic locality of Gelli-grin, 

 where the Allt-Ddu Beds and all the members of the Gelli-grin Ash 

 Series are well seen. Just as on Bryn-cut, small scarps mark the 

 position of the harder beds ; but here it is the Pont-y-Ceunant Ash 

 that makes the most definite feature, forming a low wooded ridge, 

 easily detected at the lower level on the north side of the hill 

 where it has been shifted by a tear-fault. The flatter open ground 

 above is occupied by the Calcareous Ash, which has a well-marked 

 mudstone-band near the middle of its course, and above again a 

 steep cliflc-like rise marks the position of the famous Gelli-grin 

 Limestone (Bala Limestone). In places this is a ver}'- massive and 

 highly crystalline rock, but elsewhere it consists of large concre- 

 tionary masses running together, considerable variation in this 

 respect being noticeable in the different quarries along the outcrop. 

 Upwards, however, both the more massive and the more concre- 

 tionary types pass over into thin bands of small concretions, 

 grading up into regularly-bedded, highly-calcareous mudstones with 

 a few bands of limestone, the total thickness not exceeding 25 feet. 

 The concretionary and massive types of the limestone weather to 

 a somewhat rough texture, with a conspicuous rusty-red colour ; 

 while the less massive u]3per beds show weathering of the honey- 

 comb type. Lnmediately above, and continuing the sharp rise of 

 the hill, are seen the pasty Rhiwlas Mudstones (fig. 5, p. 154). 

 The most fossiliferous parts of the limestone appear to be in the 

 quarr}'' lying west of Gelli-grin farmhouse, and along the cliff- 

 face west-south-west of the farm, this last being the exposure 

 recorded by Jukes. The fossils noted are as follows : — 



