166 



Prof. J. Prestwich. 



After eliminating those observations where the results are affected 

 by some of the various causes of interference, it would appear that 

 the sedimentary rocks in situ do not possess a lesser power of conduc- 

 tion than the igneous and crystalline rocks which underlie them, so 

 that the rate of increase of temperature need not on this account be 

 less rapid. This is especially a conclusion which may be drawn from 

 the deep (4172 English feet) and remarkable boring of Sperenberg, 

 where the bore-hole first passed through not quite 300 feet of gypsum, 

 and then entirely through rock salt of which the conductivity is 

 extremely high (0'0128), exceeding that of any other rock ; never- 

 theless, the rate of increase is 51*5 feet per degree F.* Had it been 

 a rock of lesser conductivity the rate of increase could hardly be 

 otherwise than more rapid. 



Assuming a uniform rate of 1° F. for every 48 or 50 feet of depth, 

 the heat at a depth of 28 to 30 miles would be such as to fuse the 

 basic rocks, and this has often been taken as a measure of the 

 probable thickness of the earth's crust. But there is the uncertainty 

 just named whether the rate may not increase, while the phenomena 

 before named indicate that this even is too great a thickness to be in 

 accordance with observed geological facts ? 



May it not also be a question whether, as I have before suggested, 

 the intense cold of the Glacial period has not so affected the outer 

 layers of the earth's crust that to a certain depth the rate of cooling 

 is now abnormally slow, owing to the excessive refrigeration the shell 

 then underwent. If that be the case, would not the rate of increase 

 of temperature at greater depths be more rapid than that which our 

 observations on the chilled layers have led us to assume, so that the 

 thickness of the crust might be even less than the 28 to 30 miles just 

 named ? Such a conclusion would be more in harmony with geological 

 phenomena, as we shall proceed further to show. 



The third objection to a thick crust is the difficulty of reconciling 

 such a condition of things with the effects of volcanic action. This 

 branch of the subject may be divided into the phenomena connected — 

 lstly, with recent volcanoes ; 2ndly, with the great outwellings of 

 trappean rocks during the later Tertiary period ; and, 3rdly, with the 

 character of the changes of level in the areas so affected. 



The first point relates especially to the apparent impossibility of a 

 column of lava traversing a crust 800 to 1000 miles thick in con- 

 sequence of the enormous pressure required, and without the loss of 

 so much heat in such a length of passage as to cause the lava to lose 

 its fluidity and consolidate before it could reach the surface. 



To meet this difficulty Mr. Hopkins suggested that the solid crust 

 contained at various depths beneath the surface cavities filled with 



* " Brit. Assoc. Eeports " for 1876. 



