534 PROF. T. M'K. HUGHES ON SOME 



We have to ask how far temperature and other climatal conditions 

 may have affected the amount of carbonic acid in the air and the 

 rapidity of growth and decomposition of the vegetation. 



Carried through the deep fissures in solution, as a bicarbonate, the 

 dissolved portion of the limestone is again thrown down when the 

 water emerges and is aerated or evaporated, and again offers 

 tempting problems to the speculative who try to give us a geological 

 measure of time from the rate of growth of that most irregular and 

 variable of deposits, the stalactite and stalagmite of our caves. 



In the case of a mass of rock on ice, we know that a slab so thin 

 that the sun warms it through melts its way down and sinks into 

 the glacier ; but a thick piece of rock protects the underlying ice 

 from the sun's rays, while the surrounding part is melted away, and 

 eventually the rock stands on a pedestal of ice. So in the case of 

 the earth-pillars near Yiesch or Botzen, the operation depends upon 

 the locality being to a certain extent protected from side winds and 

 upon the base of the pillars being easily and rapidly drained in 

 such a manner that they are not undermined. Where earth is pro- 

 tected from the straight downpour of rain by an umbrella-like capping 

 of stone, while the surrounding part is washed away, there an 

 earth-pillar is produced. 



So we must explain the pedestal boulders of Cunswick, Farleton, 

 and Norber. But in this case the boulder does not merely protect 

 the underlying rock from the mechanical action of the rain or cut 

 off the direct rays of the sun ; we have here to take account also of 

 chemical and organic action. 



It has been ascertained that even pure water will dissolve two 

 grains per gallon of carbonate of lime, but the water generally 

 contains a varying quantity of carbonic acid caught in the atmo- 

 sphere. As soon as specks of vegetation fix themselves upon the 

 rock the conditions are entirely altered ; not only is there a much 

 larger quantity of carbonic acid, derived from the decomposing 

 plants, but the growing buttons and masses of lichen and moss hold 

 the water like a sponge, fretting the stone away so that it has some- 

 times been mistaken for pholas-bored rock. 



A large boulder placed upon the flat limestone rock keeps off the 

 rain which would destroy the glaciated surface. It further, by 

 cutting off moisture, interferes with the growth of vegetation, and 

 by keeping the rock dry prevents its being broken up by frost ; and 

 so these pedestals of limestone seem to have an origin analogous to 

 that of the ice-pedestals on the glaciers and the earth-pillars of 

 Botzen. 



It is probable that the top of the pedestals on which the boulders 

 rest does represent a surface which has been exposed ever since 

 glacial times. But in the present state of our knowledge what 

 measure of the rate of waste can we apply to them ? or to what can 

 we apply them as a test of age ? 



The pedestals seem to be gradually perishing; in fact there 

 must be a natural limit to their height. An umbrella held too high 

 wiU only shelter from perfectly vertical rain ; so when the pedestal 



