C. EB. Dutton on the Contractional Hypothesis. 119 
questioned, and it has been argued that rocks, porous and 
saturated with water, are much worse conductors of heat than 
b 
determined by the value ofthe surface rate of increase, and to 
extend the duration of the cooling. 5 : 
Another serious quantitative modification will appear possible 
when we inquire as to the value of for places near the sur- 
. 
otic masses, are 
therefore accidental and should be excluded from arenes 
would 
is here expressed for the smaller rates, it is yet immaterial, so 
far as the present argument is concerned, which of the extremes 
be taken. 
perature, 7,000° F, now exhibiting an increase of tts 0 
gree per foot of descent near the surface. What is the present 
very gradual at first, but becoming more and more rapid, till it 
reaches the present mean temperature at the surface. 
