Coffey and Praegeii — llie Antrim Hai-sed Beach. 155- 



period of sedimentation. Now, the thickness of the Larne deposits — 

 30 feet of inter-tidal or shallow-water deposits — shows that even at 

 the point of maximum depression X, the surface of the heds was, at 

 X\ only 2 feet below high water, and became land as soon as the 

 movement of elevation set in. Further, the Larne section consists 

 first (at base) of coarse gravels (probably intertidal), then littoral 

 .sands and shallow-water Estuarine Clay (probably laid down at or 

 a little below low water), and then a thick mass of beach-gravels. 

 Constructing a curve from these facts, as in the Belfast section, we get 

 J/PiYas representing the surface of the Larne beds during deposition, 

 P being the lowest point, when the bed of Estuarine Clay was laid 

 down. Collating this diagram with fig. 1, it is seen that, as already 

 stated, the whole Larne series corresponds in age with the Estuarine 

 Clay of Belfast, and was deposited during the latter part of the down- 

 ward movement up to the point of maximum depression. 



In figs. 1 and 2, Plate V., the level of the sea is assumed as 

 constant, and that of the land as fluctuating. If we assume instead; 

 the level of the land to have been constant, and that of the sea to have 

 been fluctuating, the diagram will take a different and interesting 

 form, shown to the same scale in fig. 3, Plate Y. The assumption 

 of an even rate of sedimentation will then give the line A'F' as repre- 

 senting the increasing thickness of the beds, each of which may be 

 drawn in as shown. The vertical distance from any point on this line- 

 to the curve gives the depth of water (a negative quantity between A 

 and B) at the time. The horizontal line AF shows the position (in 

 this case one of rest) of the same point as chosen in fig. 1 — namely, 

 the base of the peat. 



Summary. 



To summarize the geological evidence. Far back in post-Glacial 

 times, the land stood relatively higher than at present. The amount 

 of this elevation was, at the time of the formation of the Belfast peat- 

 bed, still at least over 30 feet above the level now prevailing. A 

 long period of submergence ensued, during the latter part of which 

 the Larne series was deposited. The total amount of this subsidence, 

 as shown by independent evidence at Belfast and at Larne, cannot 

 have been less than 55 feet ; and it resulted in a land-level at least 

 25 feet lower than at present. We are faced by a difficulty here, as 

 no 25- feet shelf has been so far detected in the district correspond- 

 ing with this depression ; but nevertheless the evidence lent by tha 

 fauna at Belfast, and the level of the beds at Larne, appears to admit 



I 



