532 PROF. T. m'k. hughes on some 



intercepted by some of the great Silurian boulders, and the result is, 

 that the original face of the limestone has been preserved under 

 them, while all around it has been eaten away by the rain-water, 

 and so the boulder stands on a small pedestal of irregular shape, 

 according as the surface has been more or less protected from the 

 splash and wind-blown rain. We can generally see under some part 

 of these overhanging Silurian blocks, and there we find the limestone 

 smoothed, polished, and strongly furrowed and striated down the 

 valley. Thus we have the print of the old glacier stereotyped as it 

 were in the solid rock, and one good fact clearly recorded to help us 

 to work out the history of the past. 



" Another question which naturally suggests itself is, how much 

 of the limestone has been thus carried away by the rain ^only, and 

 of course the height of the limestone-pedestals above the surrounding 

 part gives us a measure of this. It appears to be generally from 12 

 to 18 inches. Here, again, we get more data for determining the 

 absolute age of some of these phenomena. Assuming the average 

 periodic rainfall to have been constant, or at any rate to be deter- 

 minable, and the quantity of limestone removed by a given quantity 

 of rainwater to be known, to find how many years have elapsed 

 since this limestone was first exposed to subaerial denudation"*. 



Mr. Tiddemant also has referred to the manner of occurrence of 

 the boulders on Norber Brow ; Mr. Davis J mentioned them ; and 

 they have been more fully described by Messrs. Davis and Lees§. 

 Mr. Mackintosh || took up the problem above stated, and offered a 

 numerical estimate of the time which has elapsed since the limestone 

 over which the boulders are scattered was exposed to denudation. 

 As I pointed out in the discussion on that paper, there were too many 

 unsupported assumptions to allow us to attach much importance to 

 the results at which he arrived. 



The value of this kind of evidence depends upon the accuracy of 

 the observations on the manner and rate of waste of the particular 

 beds of limestone on which the boulders rest. Every one familiar 

 with the Mountain Limestone knows how some parts stand out in 

 bluffs and some are readily cut back by subaerial weathering. Such 

 overhanging ledges and such hollow places, the rock-shelters of 

 primaeval man, are seen in most limestone districts, whether in the 

 newer rocks of the Dordogne or the older rocks of the Elwy and of 

 Parleton Knot. The mode of weathering is determined by small 

 differences in the character of the rock, such as the tendency to 

 break up into thinner beds, the quantity of earthy matter, the 

 crystallization being uniform and complete through great masses, 

 or producing only small concretionary nodules. 



* " Notes on the Geology of Parts of Yorkshire and Westmorland, by T. M*" K. 

 Hughes, M.A., F.a.S.," Proe. Geol. Polytechn. Soe. West Riding of Yorkshire, 

 July 1867, vol. iv. p. 574. 



t Quart. Journ. Geol. Soc. vol. xxxviii. 1872, p. 477. 



J Proe. Geol Polytechn. Soc. W. Eiding Yorkshire, vol. vii. 1880, p. 266. 



§ West Yorkshire, pp. 200, 201, 267. 



1 Abst. Proe. Geol. Soc. Feb. 21. 1883, p. 67. 



