TRANSACTIONS OF THE SECTIONS. 87 



At this station, wherever the rock is exposed to view, water will be found also, 

 oozing out here and there from the face of the cliif between the glacial clay and the 

 underlying rock. Where this occurs, abundant specimens of the corroded limestones 

 are to be met with ; and no doubt if the passage of water were sufficiently pro- 

 longed, it would in time lead inevitably to the formation of gravel-beds between 

 the clay and the rock. But the percolation which was observed appears to be of 

 too modern a date. It probably began after the existing cliif was formed by the 

 advance of the sea. 



A case, however, in which Upper Glacial Drift has imdoubtedly been con- 

 verted into Middle Drift, will be found at the south end of the Ladies' Bathing- 

 place at Greystones. Here the upper part of the cliff consists of glacial 

 clay with a few striated boulders, and below it there are imperfectly stra- 

 tified beds of sand and gravel. These porous beds communicate with the surface- 

 soil by means of a chimney-like passage filled with gravel, which is exposed in the 

 cliff, and through which rain gained access to the beds below. These beds bear 

 evidence that, before they were cut across by the sea, they formed part of a sub- 

 terranean reservoir, in which slowly flowing water was confined between the rocks 

 beneath and the glacial clay above, the lowest layer of the glacial clay to the 

 depth of several inches having acquired a structm-e which is stratified parallel to' 

 its under surface. This stratified structure is continued along the clay waUs of the 

 passage by which the rain had access to the porous beds. In this case the corroding 

 and transporting action of the water upon the underside of the glacial di-if't has 

 probably oeen suspended since the subteiTanean reservoir was tapped by the en- 

 croachment of the sea ; but the appearances clearly show that it had before that 

 time been going on, that the water in its passage had been con-oding or carrying 

 away some constituents of the Upper Glacial Drift over its under surface, and that 

 the residue which remained became an accession to the underlying beds of gravel 

 and sand. 



These particular spots have been pointed out, because they can be easily identified 

 by other observers ; and abundant confirmatory evidence will be found all alou"- 

 the three miles of coast examined by the author. In some places isolated masses 

 of clay will be found in the gravel-beds, which may be presumed to be outstanding 

 portions of the glacial clay ; and in all places the ratio of the limestones to the 

 other kinds of stone is much less in the gravel-beds than in the glacial clays, which 

 is the state of things that would natm-aUy arise if the gi-avel-beds have been wholly 

 formed out of glacial deposits by the prolonged action of the causes which we find 

 still in operation. 



There is also evidence to show that the stratification of the gravel-beds is due to 

 causes still existing ; for in the places where there is the most immistakable evi- 

 dence that Middle Drift is to this day being formed at the expense of glacial drift, 

 it was found that the stratification of the gravel-beds was continued almost into 

 contact with the glacial clay, and therefore through the parts most recently con- 

 verted. But this need present no difficulty. An old mill-race, with a mound alono- 

 one side of it, has lately been removed to make way for the extension of Dublin 

 towards Sandymount. The mound when cut across presented as fully developed a 

 stratified structure as is usually seen in natural gravel-beds. And what made the 

 case conclusive was that the distribution of the materials was in most places wholly 

 different both in kind and degree from what the original piling of tliem together 

 coidd have occasioned. It must therefore have been produced subsequently, and 

 it was interesting to observe most of the main features of natural stratification re- 

 appearing in it, with their unconformable beds, separation of fine from coarse mate- 

 rials, and so on. 



A very simple experiment will show how much may be effected even in a short 

 time by the percolation of water through gravel-beds. Pour a few cans of water 

 upon sand which consists of particles of various degrees of fineness, and if a little 

 clifi" be then made in the wet sand, it will be found to be already stratified in a very 

 considerable degree. 



