On the Moel-Tryfan Shelly Deposits, Sfc. 71 



15 yards, and at the other end 5 yards. The author estimated that 

 the deposit of sand that had accumulated in 8 years amounted to 

 an average of 10 yards width and 2 yards depth. Allowing a further 

 depth of 1 yard for sand that may have been blown over the top, he 

 finds 10,500 cubic yards as the quantity of sand deposited in 8 

 years on a shore-frontage of 350 yards, or 3-75 cubic yards per yard 

 of frontage per annum. Applying this unit of measurement to the 



16 miles of coast forming the western boundary of the deposit, he 

 gets 105,600 cubic yards as the quantity of sand annually moved ; 

 22 square miles of sand, 12 feet thick, give 272,588,800 cubic yards 

 of sand accumulated, which, divided by the annual quantity, will 

 give 2580 years as the age of the whole deposit of blown sand. The 

 author adduced other evidence in support of his view, and concluded 

 that if the last change of level in South-west Lancashire was a 

 downward one, it could not have taken place within 2500 years. 



April 27. — Kobert Etheridge, Esq., E.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. "On the precise Mode of Accumulation and Derivation of the 

 Moel-Tryfan Shelly Deposits ; on the Discovery of similar high-level 

 Deposits along the Eastern Slopes of the Welsh Mountains ; and on 

 the Existence of Drift-zones showing probable Variations in the Rate 

 of Submergence." By D. Mackintosh, Esq., E.G.S. 



The author commenced by giving a sketch of the progress of dis- 

 covery connected with the Moel-Tryfan deposits. He then described 

 certain phenomena connected with these deposits, to which little or 

 no attention has been devoted by other observers. After identifying 

 the local stones and indirectly local erratics, he traced the derivation 

 of the far-travelled erratics which came from the IN", and N.W, He 

 drew particular attention to an extensive exposure of slaty laminae, 

 the edges of which have been bent by a force assailing the hill from 

 the N.W. ; and as these edges have been shattered so as to form 

 parcels of slate-chips covered by or rolled up in laminated sand, 

 along with parcels of clay, he endeavoured to prove that a stranding 

 of the floating ice which must have brought part of the erratics 

 (including numerous chalk-flints) will alone account for the pheno- 

 mena. After describing patches of gravel and sand in other parts 

 of Caernarvonshire, referring to the Three-Rock Mountain deposits 

 in Ireland (which must have come from the N.W.), and briefly 

 noticing the drifts on Halkin Mountain, Elintshire, he entered upon 

 the main subject of his paper, namely the discovery of an extensive 

 series of marine drifts, including (besides deposits on flat ground) 

 about twelve hillocks or knolls, consisting of rounded gravel and 

 sand, and, in at least two instances, containing gravel- pits with nu- 

 merous shell-fragments. They extend along the east side of the 

 northern part of the mountain-range which runs between Minora 

 and Llangollen Yale, and are situated at levels between 1100 and 

 1300 feet above the sea. The gravel is largely made up of rounded 



