86 REPORT — 1870. 



consisting of subangular and rounded fragments from the Coal-measures and the 

 Triassic sandstone, as well as liver-coloured and quartz pebbles from the Middle 

 Bunter, and likewise granite, porphyrj-, greenstone, and other pebbles derived from 

 the Boulder-clay, one of which affords an excellent example of glacial markings. It 

 may therefore be inferred that these fissures must have been formed during the recent 

 or at least the postglacial period. 



Mr. 0. Smith exhibited an Orthopterous insect. 



On the recent Formation of Gravel-beds resemhl'mg Middle Drift. 

 By G. Johnstone Stonet, M.A., F.'li.S, 



- On the east coast of Ireland, extending south of Dublin from Killiney to Bray 

 Head, and from Bray Head to Greystones in the coimty Wicklow, there are consi- 

 derable cliffs of Drift exposed to view. They consist of (1) masses of amorphous 

 glacial clay, usually containing an abundance of striated limestone boulders and 

 fragments of shells ; (2) gravel, which is almost unstratified, containing a few stri- 

 ated boulders and fragments of shells ; and (3) stratified beds of gravel and sand, 

 without striated boulders and with but very few fragments of shells. The strati- 

 fied beds have usually been regarded as Middle Drift, the other beds being treated 

 as Upper Glacial Drift or Lower Glacial Drift, according to their position. 



The author had carefidly examined these formations from Bray Head to a little 

 south of Greystones, and had satisfied himself that the undoubtedly glacial deposits 

 are to the present day in process of being transformed into the stratified beds of 

 gravel and sand, which have been usually referred to Middle Drift. 



The Drift rests directly on argillaceous slates of the Cambrian formation ; and the 

 first thing which attracted attention was that all the spring water of the neighbour- 

 hood is hard, containing an abundance of salts of lime. These could onlj^ have 

 been obtained from the limestones of the Drift, and consequentl)^ indicated that a 

 sensible amount of change is going on in it. This led the author to investigate 

 further, and he then found evidences of change everywhere and unmistakable. 

 But it will be most useful to confine this record to the appearances in two situa- 

 tions, which can be without difficulty identified by any person who may wish to re- 

 examine the groimd. 



Under the railway station at Greystones, which is built close to the shore, a cliif 

 of glacial clay with abundance of striated boulders will be found. It rests directly on 

 the Cambrian slates, which are also exposed to view. Immediately in front of the 

 railway station the clay reaches to the surface of the ground ; but 80 or 100 yards 

 further south beds of stratified gravel wall be found above it, the stratification being 

 usually oblique to the sm-face of contact of the gravel and clay. Here it will 

 be easy to see evidence that rain when it falls soaks quickly through the 

 gravel, and then travels along the upper sm-face of the clay. In doing so it 

 penetrates the clay to a certain distance, from 2 to 6 inches, altering its colour from 

 a dirty blue, which is the prevailing colour of the glacial clay at this spot, to a light 

 fawn colour. If this stratum of clay out of which the colour has been discharged 

 be dug into, the remains of limestone boulders will be found scattered through it, 

 which have also been attacked by the water. There usually remains either a 

 powdery mass of the insoluble matter of the boulder, or such a mass with a core 

 of some fantastic shape, which is the part of the limestone that has not yet been 

 dissolved away. In a few instances specimens were foimd in which one side of the 

 stone had been sheltered from the water, and still retained its glacial markings. 

 Moreover some of the clay is washed forwards when the water travels over it, for 

 wherever the water dribbles out at the surface of the cliff' it carries clay out with it. 

 Here, then, we have an instance in which the so-called Lower Glacial Drift is being 

 in part corroded or washed away all over its upper sm-face ; and the portions that 

 remain are being added on to the lower surface of the overljdng IMiddle Drift. The 

 part which is being converted into Middle Drift appears to consist principally of the 

 insoluble stones of the glacial clay, and of such central portions of the larger lime- 

 stones as may outlive the corroding process. 



