T. Me/lard Reade — Oscillations of Level of Land. 491 



stripped from the Triassic rocks, we should behold a landscape of 

 a much more varied character than that which we now see. 



If, as I have endeavoured to show, the geological facts related 

 prove the existence of three land- surfaces, the first in time being 

 pre-Glacial or, at all events, pre-Boulder-clay, and cut in the Triassic 

 rocks, the second post-Glacial, represented by the buried eroded 

 surface of the Boulder-clay, and the third indicated by the peat and 

 forest beds, it is evident that there have occurred a good many 

 variations in the relative levels of land and water. In the first 

 or pre-Boulder-clay period, taking into consideration the known 

 depth of many buried valleys, it is highly probable that this was 

 a Continental period, with all the British Islands, united, and the 

 various rivers converging into one large river traversing the basin 

 now occupied by the Irish Sea, and delivering its waters into the 

 Atlantic through the Straits of Galloway, where there is a " broad 

 ditch " 600 feet below the general level of the bottom. The land 

 then sank beneath the waves, and the marine Boulder-clay was 

 deposited. 



Certain supporters of land-ice as against every other cause 

 maintain that there was no alteration of the relative level of land 

 and sea at this time, but that the low-level marine boulder-clays 

 and sands and the high-level sands and gravel are simply sea-bottom 

 swept on to the land by an ice-sheet advancing from Scotland over 

 the Irish Sea bed. Suppose we grant this contention, which 25 years' 

 observations has led me to reject, the pre-Boulder-clay valleys 

 running down to a level far below low-water remain unaccounted 

 for, and it is necessary still to postulate a pre-Boulder-clay elevation 

 of the land to account for these river-channels eroded in the Triassic 

 rocks. If we admit this reasoning, and I see no escape from it, the 

 a priori assumption against the probability of a movement of sub- 

 mergence during the Glacial Period, of which, 1 maintain, the whole 

 of the marine glacial deposits up to 1400 feet are indicative, is 

 knocked off its feet. The first period of oscillation, then, is repre- 

 sented by a high-level land-surface, followed by a subsidence 

 of a river- valley system below the waves, and, I maintain, by a still 

 further depression of 1400 feet during the deposit of the low-level 

 marine boulder-clays and high-level sands and gravels containing 

 marine shells. 



The second period of oscillation represents an emergence of the 

 sunken land from its extreme depression to a level above the sea 

 considerably greater than we see now. In a minor manner stream 

 and river valleys were cut in the Boulder-clay similar to those of 

 the previous period. In the third and last period of oscillation 

 a depression of the land again occurred to the 25-feet contour, and 

 the post-Glacial silts and Scrobicularia clays were laid down upon 

 the eroded surface of the Boulder-clay with its minor valley system. 



Again a reverse movement took place, the land being elevated 

 above its present level, allowing the growth of forest trees now 

 swept by the tide, and finally these were submerged by the last 

 downward movement of the land. 



