654 THE POPULAR SCIENCE MONTHLY. 



the family life will develop the more internal unity of the married 

 twain and the more perfect moral nourishment of their offspring will 

 bring on a civilization which must write itself out in " sweeter man- 

 ners, purer laws." 







KOCK-WEATHERmG, AS ILLUSTRATED IN CHURCH- 

 YARDS.* 



By Professor AECHIBALD GEIKIE, F. R. S. 



COMPARATIVELY little has yet been done in the way of precise 

 measurement of the rate at which the exposed surfaces of differ- 

 ent kinds of rock are removed in the processes of weathering. A 

 few years ago some experiments were instituted by Professor Pfaff, 

 of Erlangen, to obtain more definite information on this subject. He 

 exposed to ordinary atmospheric influences carefully measured and 

 weighed pieces of Solenhofen limestone, syenite, granite (both rough 

 and polished), and bone. At the end of three years he found that the 

 loss from the limestone was equivalent to the removal of a uniform 

 layer 0'04 millimetre in thickness from its general surface. The stone 

 had become quite dull and earthy, while on parts of its surface fine 

 cracks and incipient exfoliation had appeared.f The time during 

 which the observations were continued is, however, too brief to allow 

 any general deductions to be drawn from them as to the real average 

 rate of disintegration. Professor Pfaff relates that during the period 

 a severe hailstorm broke one of the plates of stone. An exceptionally 

 powerful cause of this nature might make the loss during a short 

 interval considerably greater than the true average of a longer period. 



It occurred to me recently that data of at least a provisional value 

 might be obtained from an examination of tombstones freely exposed 

 to the air in graveyards in cases where their dates remained still 

 legible or might be otherwise ascertained. I have accordingly paid 

 attention to the older burial-grounds in Edinburgh, and have gathered 

 together some facts which have, perhaps, sufficient interest and nov- 

 elty to be communicated to the Society. 



At the outset it is of course obvious that, in seeking for data bear- 

 ing on the general question of rock-weathering, we must admit the 

 kind and amount of such weathering, visible in a town, to be in some 

 measure different from what is normal in nature. So far as the dis- 

 integration of rock-surfaces is effected by mineral acids, for exam- 

 ple, there must be a good deal more of such chemical change where 

 sulphuric acid is copiously evolved into the atmosphere from thou- 

 sands of chimneys, than in the pure air of country districts. In these 



* A paper read to the Royal Society of Edinburgh, on April 19, 1880. 

 f " Allgemeine Geologic als exacte Wissenschaft," p. 317. 



