154 



Proceedings of the Royal Irish Arademf/. 



from the middle of each Led we drop a coordinate to the point which 

 represents the average level (either depth of water or li eight above 

 sea) at which, according to its fauna and other characteristics, we 

 believe each bed to have been deposited. The movements of any 

 point at Alexandra Dock relative to sea-level are then represented by 

 the curve drawn as a firm line, the particular point chosen in this 

 case being the base of the peat, where the post-Glacial series begins. 

 The movements of the surface of the ground do not correspond with this 

 curve, since the surface has kept rising, owing to accumulation of 

 sediment ; the dotted line represents the surface-level throughout the 

 period in question.^ The method is, of course, very rough-and-ready, 

 but nevertheless shows the main features of the fluctuations under 

 discussion. Viewed then according to the scale of accumulation at 

 Alexandra Dock, the curve shows the fluctuations of level in the 

 neighbourhood of Belfast. Assuming, as we have already done, that 

 the same fluctuations of level occurred at Larne, we can apply this 

 curve to the Larne section with interesting results (fig. 2, Plate Y.). 

 Here, on the same scale, the line ZZ' represents the Curran section at 

 the present day, from the top to the surface of the Eoulder-clay, 

 adjusted to its present position as regards sea-level. The curve TF"^ will 

 represent the fluctuation of the point Z (the surface of the Boulder- 

 clay), according to the standard of the Belfast section. If we draw 

 from Z' a parallel curve Z'Y'X', this represents the simultaneous 

 movement of the surface of the gravels until they dip down below 

 high water at JV. Working from the other end of the curve, we find 

 that the base of Boulder-clay remains above the sea up to the point 

 M. Therefore the deposition of the Larne gravels cannot have begun 

 earlier than 3f nor continued later than JV; the gravels were deposited 

 during the downward movement of the land represented by the curve 

 MX. It follows that some line joining J/ to will represent the 

 highest level that the surface of the deposits could have liad during the 



1 Leaving out of account, of course, any denudation that may have taken 

 place, concerning- which evidence is not available. The sharp line of demar- 

 cation between the top of the deep-water clay and the overlying beach-sand 

 (line 5) undoubtedly represents denudation, and consequently time, perhaps of 

 considerable amount ; and the thickness of the beds at the close of the deposition 

 of the upper clay (represented by that portion of line 5 which lies between the 

 two curves) may have been, greater, subsequently diminishing by denudation 

 towards U, till finally, by the deposition of the beach-sand, the depth of deposits 

 became JiJE'. 



