Vol. 62.] THE CARBONIFEROUS ROCKS AT RUSH. 293 



of Giant's Hill. Following these beds to the seaward face of 

 Giant's Hill, another good section in the Cyathaxonia-Be&s is 

 exposed. The rocks are here arranged in an asymmetrical anticline, 

 the northern slope of which as far as Brook's End is vertical, 

 while the southern has a dip of only 35° to 45° ; and in the core of 

 the arch are several minor folds with a tendency to be overfolded 

 or overthrust towards the north. The ' augen '-looking character 

 of the thin limestones is well marked in many of the beds here. 



Returning now to the Bathing-Place, we can walk at low tide to 

 the rocks seaward of it, and make a traverse to the north parallel 

 with the section in the cliff. The beds are somewhat overturned at 

 the commencement, but the structure is practically a simple syncline 

 of which the central beds are unexposed through the drifting-in of 

 sand. We begin with a horizon that can be followed to the cliff, 

 and we end with a horizon that can be traced into the cliff; but 

 the intermediate beds of the syncline are very different in the two 

 parallel sections. Seaward, the beds are of the ordinary character 

 previously described, that is to say they are for the most part 

 limestones ; whereas all the limestones have disappeared from the beds 

 of indubitably the same horizon in the cliff-section, where all that 

 is left in lieu of the original rocks is a contorted, weathered, and 

 ironstained mass of shale and chert, representing the insoluble 

 residue of the beds after the removal of the calcareous matter. 

 These decalcified beds have a thickness of apparently about 100 feet, 

 which of course represents a much greater thickness of the original 

 rocks. On each side of the syncline enclosing the decalcified 

 material, the limestones are for some little distance rather rotten 

 and partly decalcified, but the change to complete decalcification is 

 more abrupt than would be expected. 



The excavation of limestone by underground water, the ' piping ' 

 of chalk, and similar phenomena resulting in the local removal of 

 limestone-beds, have of course long been known. The dwindling 

 and disappearance of limestones by solution was discussed, from a 

 theoretical standpoint, by Rutley in 1893 l ; and in 1903 Mr. C. T. 

 Clough 2 described actual instances of their disappearance in High 

 Teesdale, where the Great Limestone, 55 feet thick, is shown to have 

 been often completely removed (especially along the outcrop) and 

 replaced by 10 or 12 feet of soft siliceous clay ('famp'"). I have 

 myself seen further instances of limestone-solution along the Dublin 

 coast nearer Loughshinny ; and it is probable that this effect is 

 commonly produced on a large scale in many parts of Ireland. 

 Indeed, the origin of many of the great Irish lakes, and even of the 

 gulfs and bays, has been held to be the result of chemical solution 

 of Carboniferous Limestone. 3 



There are two small but interesting thrust-planes, in the black 

 decomposed shales with cherts that form the southern margin of 

 the decalcified beds described above. Their hade is very low, and 



1 Quart. Journ. Geol. Soc. vol. xlix (1893) p. 372. 



2 Geol. Mag. 1903, p. 259. 



3 E. Hull, ' Physical Geology & Geography of Ireland' 1878 ; pp. 198-209. 



