78 G. H. Kinahan — Tide Heights and Raised Beaches. 



Group 2. — ScALARiA, sub-genus Opalia, Adams. 



This division of the ScalaricB comprises by far the larger proportion 

 of species, both in Europe and elsewhere ; they are all characterized 

 by the presence of a well-marked basal keel, which is sometimes also 

 visible at the sutures. I commence by describing the best known 

 and most distinct forms from the Gault, and will then notice the 

 2>oints of difference which exist between them and the Greensand 

 s^Decies hitherto known und:er the same names. 



(Jb he continued in our next •Number.) 



Y. — Irish Tide Heights and Eaised Beaches, 

 By G. H. Kinahan, M.E.I.A., etc. 



A GOOD deal of attention has been lately bestowed on the Eaised 

 Sea-beaches around the British Islands and the evidence 

 afforded by them as to alterations in the level of the land. Some 

 geologists seem to entertain the idea that such beaches were origin- 

 ally accumulated on horizontal lines ; and they have drawn the con- 

 clusion, that when one portion is found to be higher than another, 

 it necessarily indicates an uneven rise in the surface of the earth. 

 This idea, however, is erroneous; and the object of this paper is to 

 prove, from the data supplied by the Ordnance Survey and the 

 Admiralty Charts,^ that the littoral deposits at present accumulating 

 around Ireland are being formed at varying levels. 



From the Introduction, by Captain J. Cameron, E.E,, to the 

 '^ Abstracts of the Principal Lines of Spirit Levelling in Ireland," 

 we learn that the zero plane of the Ordnance Survey in Ireland is 

 the level of low water of the spring-tide observed at the Poolbeg 

 Lighthouse, in Dublin Bay, on the 8th of April, 1837, which is 

 20*94 feet below a mark made in the base of the Lighthouse. At 

 first the different lines of levelling were referred to the low water 

 of spring-tides at various convenient places around Ireland; this, 

 however, was found, on further examination, to be so irregular, that 

 subsequently certain sections were established in different places, 

 where series of spirit-level observations were taken, by which these 

 stations were connected with each other, and thus all the means of 

 the different places were referred to one datum level, that just 

 mentioned. The results of these observations are given at page xii. 

 of Captain Cameron's Introduction, and from his table have been 

 constructed Tables No. I. II. III. and IV. 



In column i. of these tables are given the heights at each station 

 of mean water above the Ordnance zero plane, and from it we learn 

 that the height of mean water varies but slightly round Ireland, and 

 that the lowest is at Kilbaha Bay, near Loop Head, on the north 

 of the mouth of the Shannon. Column ii. gives the heights of mean 

 water at each station above the lowest, which occurs, as just stated, at 

 Kilbaha. Column iii. gives the mean height of high spring-tides 



1 Nearly all the heights hereafter given are taken from the levellings of the 

 Ordnance Survey, a few being added from the Admiralty Charts in those places where 

 the Ordnance Survey had not stations. 



