124 



Prof. J. Prestwich. 



Besides these, there are other considerations which should not be 

 overlooked, although it is impossible at present to assign a value to 

 them. Still they may be placed to a suspense account. The first is 

 whether there may not be areas of certain rocks in which the gradient 

 is more rapid than in other areas ; and whether in tropical regions 

 there is not generally a more rapid thermometric gradient. In the 

 paper on Underground Temperatures, a few of the observations 

 raise these questions, as questions for further inquiry. 



The second point, which I have already mooted,* is more purely 

 hypothetical. It is whether the effect of the excessive cold of the 

 glacial period — or cold prolonged during so many thousands of years 

 — may not possibly have left its mark on that portion of the earth 

 covered for so long a period by perpetual snow and ice, — whether the 

 loss of heat in the upper layers of the crust may not only have altered 

 the thermometric gradient, but also induced, as it were, premature 

 contraction by an excessive abstraction of heat during that period. 

 Whether also that outer portion of the crust so affected might not 

 now present a slower gradient than the present mean surface tem- 

 perature would warrant, while at greater depths a normal more rapid 

 gradient may still prevail. And whether or not this might possibly be 

 an element in the present effective rigidity of the crust ? 



Taking therefore into consideration all the conditions to which 

 water becomes subject with increasing depth and the rapid increase 

 of temperature, together with the circumstance that while the pres- 

 sure of water increases with depth in simple arithmetical progression, 

 that of the elastic vapour of water is one of a very rapid geometrical 

 progression, it becomes extremely improbable that water can penetrate 

 beyond a certain depth beneath the surface. Roughly, it is a ques- 

 tion whether 7 to 8 miles would not be a limit. At all events I feel it 

 impossible to accept any hypothesis based upon an assumed percola- 

 tion to unlimited depths, and am forced to look to other causes in 

 explanation of the presence of water in volcanic eruptions. 



It is true that the experiments of Daubree, which will be further 

 alluded to, show that owing to the force of capillarity, water can 

 pass through porous strata against a considerable resisting pressure, 

 but on the other hand Wolff's experiments show that the effects of 

 capillarity decrease with the increase of temperature, and tend to 

 prove that there is a point at which they would altogether cease. 



It may also be a question whether at the high temperature at great 

 depths, the vapour of water would not undergo decomposition, for 

 M. H. St. Claire Devillef has shown that under certain conditions, at 

 a temperature of from 1103° to 1300° C, it is dissociated into its 

 * " Phil. Trans.," vol. 164, p. 305. 



f " Sur le phenomene de la dissociation de l'Eau," " Comptes rendus," vol. lvi, 

 p. 195. 



