part 2] ITS STRUCTURE AND ROCK-SUCCESSIOIS". 141 



separated as a definite ash, retaining its coarse character, while 

 at Creigiau Bychain it does not exceed 6 inches in thickness, as 

 distinct from the Calcareous Ash. It is very variable in character 

 also, varying even in one quarry from a very coarse to an even- 

 grained I'ock of fine texture containing mudstone patches ; at 

 times, also, there is very little ashy material beyond certain blacker 

 lumps, which are, however, quite distinguishable from the mud- 

 stone ; seen under the microscope the ash is found to be largely 

 composed of a great variety of highly silicified lavas, some of 

 which are vesicular. 



Gelli-grin Calcareous Ash Series. 



The Calcareous Ash Series of Gelli-grin is the most interesting 

 series in the district, on account of its variable nature. It seems 

 to be throughout its thickness a potential limestone, and to have 

 pure limestones developed within it at different horizons in different 

 places ; these are all of a more or less lenticular character, appear- 

 ing and dying out fairly rapidl3^ Thus, at Grelli-grin, above the 

 Pont-y-Ceunant Ash, a thickness of about 40 feet of Calcareous 

 Ash occurs with an ash-free mudstone-band near the middle, and 

 above the Calcareous Ash is found the calcareous development so 

 long regarded as the typical Bala Limestone, comprising 20 feet 

 or so of massive and concretionary limestones overlain by alter- 

 nating limestone and mudstone bands; these appear to lie wholl}^ 

 above the Calcareous Ash, being at once succeeded by the 

 B-hiwlas Beds, of a totally different nature. This development is 

 continued on Bryn-cut, on the opposite side of the Hirnant valley, 

 but it changes when traced southwards. In the stream-section 

 between Gelli-grin and Moel-fryn there is no trace of any limestone 

 whatever, although calcareous ashy mud stones seem to be well 

 developed ; on Moel-fiyn to the south the limestone reappears, but 

 in the middle of the Calcareous Ash approximately at the horizon 

 of the mudstone-band of the Gelli-grin section, Avhilea considerable 

 thickness of ashy mudstone at the top seems to represent the 

 Gelli-grin Limestone. On Brjai Pig still farther south the 

 development agrees in the main with that at Gelli-grin ; but the 

 limestone proper seems to extend rather higher up into the series, 

 with a resultant greater thickness of crj^stalline limestone. On 

 Creigiau B^^^chain there is no calcareous development at all, apart 

 from the Calcareous Ash ; but immecliatel}^ south of Creigiau 

 Bychain, in the stream at the foot of the hill, is another calcai-eous 

 development — the Caerhafotty Limestone — at the ver^^ base of the 

 Calcareous Ash. No further development of limestones of any 

 thickness is found in the Calcareous Ash in the area here described, 

 though a band about 2 feet thick of an ashy limestone has been 

 worked for lime at Maes-meillion. 



In addition to these impersistent hmestones, there is a definite 

 variation in the Calcareous Ash itself, when traced from norfh to 



