'911- Stewart. — Fluchiations of Sea-level. 23 



come upon a littoral or shore deposit, a bed that is not 

 entirely local, but has been noted in several other places 

 in the British Isles, and is styled the Scrobicularia bed. 



This bottom bed differs from the overlying clays, not 

 only in the Testacea it contains, but also in physical 

 characters. It is a blackish clay, not nearly so fine-grained, 

 so tough, or unctuous as that nearer the surface. It is 

 charged with a vast number of shells, of a few species, all 

 littoral — shells in fact of Mollusca which usually live between 

 tide-marks on a muddy shore, or on mud-flats near high 

 water mark. They consist of Cockles, Periwinkles, and 

 Mussels, with Tapes decussatus, now comparatively scarce in 

 the bay, and Scrobicularia -piperata, which at present does 

 not live here, nor indeed anywhere in the North of ^Ireland. 



This Scrobicularia bed tells of a time when the sea stood 

 at a level slightly higher than at present, and when at high 

 water the Bog Meadows were covered by the tide, as also 

 the Plains, and much of the low ground on which the town 

 now stands. This level must have been constant or nearly 

 so for a long time, as the accumulation of mud amounted 

 to four or five feet more or less over a large area. It is 

 replete throughout with shells of the same species, and its 

 lithological characters are always the same. 



I consider then that_ the Scrobicularia period continued 

 for a very considerable time with little change of level, and 

 was succeeded by a period of depression. This constitutes 

 my third period, the period when Thracia convexa was 

 dominant. The rate of this depression was rather quick ; 

 at least the sinking, if not rapid, was at any rate not of 

 the nature of those slow movements or supposed movements 

 attributed to historic times about which there has been so 

 much discussion. 



3. The advent of the Thracia convexa period was 

 marked by a rather abrupt change from a blackish clay to 

 an extremely smooth tenacious clay which was light blue or 

 slate-coloured. The altered character of the clay indicates 

 a change of the conditions of deposition, and was 

 accompanied by a great change in the fauna. The littoral 

 shells which had exclusive possession of the bottom 

 disappear, and are replaced by shells of species that are 



