So4 



Mam — Tufa Deposits in Flintshire. 



bearing tlie impressions of Equisetums, Carices, and other marsh 

 plants; also, occasionally, willow leaves. The remains of recent 

 shells are most abundant — Helix memoralis occuning in great 

 quantities throughout the deposit. 



Mr. Grwyn Jeffreys has also determined the following species : — - 



Limncea peregra (Miill.) 

 Succinea putris (Linn.) 



* Zonites nitidulus (Drap.) 



,, crystallinus (Miill.) 

 „ ficlvus (Miill.) 

 ,, radiatulus (Aid.) 

 Selix aculeata (Miill.) 



* ,, aspersa (MiiU. 



* „ nemoralis var. hortensis (Pern.) 



* „ arbustortimxax.alpestrisiZviglGr:.) 



Helix concinna (Jeffreys.) 



,, caperata var. Gigatii (Charp.) 



,, rotimdata (Miill.) 



11 pygmcRa (Drap.) 



„ pulchella (Miill.) 

 Vertigo antivertigo (Drap.) 

 Zua {Conchlicopa) lubrica (Miill. sp.) 

 Carychium oninimum (Miill.) 

 Cyclosloma elegans (Miill.) 



The species marked thus * were obtained between Pwll Gwyn 

 and Caerwys ; all the others and some of those marked *' from the 

 railway-cutting, near the Smell turnpike-gate. 



Mr. Jeffreys observes that all the species at present inhabit Wales, 

 and, probably the locality where found; and considers the deposit 

 an extremely modern one. Some of the species indicate a moist 

 situation, others a dry soil. 



In a former communication to the Magazine (Yol. II., p. 200), 

 I described the occurrence of curious cavities or pockets in the 

 Momitain Limestone of the same district, which I suggested 

 had been formed through the gradual dissolution of the limestone, 

 by a process similar to that which has been supposed to explain the 

 method of formation of sand-pipes in the chalk. The occurrence of 

 the redeposited lime as Tufa in the immediate neighbourhood affords 

 another point of analogy between the limestone-pockets and the 

 chalk sand-pipes, and further supports the theory suggested to 

 account for their excavation. 



The subterranean derivation of the Tufa is here also indicated by 

 the existence of a cavernous opening in the limestone, lined with a 

 stalactitic coating immediately over the upper end of one of the 

 Tufa beds (see Section) ; and it seems probable that from this cavity 

 the water that deposited the Tufa once flowed. The cavern is now 

 dry, and the deposition of the Tufa appears to have entirely ceased ; 

 the only water that flows over it occurs as a stream that has its 

 origin higher uj) the valley, and instead of adding to the accumula- 

 tion of the deposit it has cut a channel throiigh it, in some places to 

 a considerable depth. These facts seem to imply a change of cir- 

 cumstances from those which obtained when the Tufa was originally 

 deposited, and the present stream, if it existed at the time, must 

 have occupied a different position. 



Assuming that the various cavernous openings in the Moun- 

 tain Limestone have been similarly formed, it appears probable that 

 these subterranean excavations proceeded during a long geological 

 period ; the caves of Cefn, on the opposite side of the vale of 

 Clwyd, appear to have been excavated before the Boulder-clay-driffc 



