On the Agency of Water in Volcanic Eruptions. 1 63 



mass, unless it all took place at once after the deposition of the over- 

 lying strata was ended, which is impossible, would be pro tanto to 

 diminish the rate and extent of subsidence. Further, alterations of 

 level arising from this cause are not likely to have taken place in 

 Glacial and Post-Grlacial times, as the sedimentary matter then depo- 

 sited was generally limited to a few irregular masses of sand, gravel, 

 and shingle, or to a few raised beaches and shell beds only a few feet 

 in thickness ; while the submarine beds rarely reached a thickness of 

 more than 300 or 400 feet. 



The next point we have to consider is, that of the increase of tem- 

 perature with the increase of depth. On this, there is a general 

 agreement as to the fact, but a difference of opinion as to the rate of 

 increase and in the inferences to be drawn from the observations. The 

 rate of increase is found to differ very materially at different places, 

 varying in round numbers from 30 to 100 feet for each degree F., 

 or with a mean by some taken at 50 and by others at 60 feet ; and 

 conclusions have been drawn by some geologists from these observa- 

 tions that the nucleus of the earth is in a state of fluidity at a depth 

 of 30 miles, whereas others have considered the facts not incompatible 

 with a crust of 800 miles or more thick. 



The variation in the rate of increase with depth is influenced not 

 only by the variable conductivity of the strata, but depends also upon 

 so many disturbing causes, that while all the observations tend to 

 prove the general fact of an increase of temperature with depth, very 

 few can be relied upon to give an exact measure of that increase. 

 Notwithstanding the care taken by some of the earlier and all the 

 later observers against these disturbing causes, these are so numerous 

 and often so difficult, if not impossible to guard against, that only a 

 limited number of observations can be relied upon to give the exact 

 data required. I have gone elsewhere* into the various considerations 

 affecting this question, so that they need not be repeated here. The 

 conclusion that I have arrived at is, that the observations that can be 

 best relied on show an increase of not less than 1° F. for every 48 feet 

 of depth, and which there is some reason to believe may be even more 

 rapid . 



Whether or not the rate of increase diminishes or increases with 

 the depth is a question which requires further investigation. The 

 few observations that have been made throw little light on the 

 subject. These are given in the paper before referred to. They 

 certainly do not tend to prove that there is any diminution in the 

 rate of increase of temperature. There are some reasons on the 

 contrary which would lead to an opposite opinion. 



Ante, p. 1. 



