78 president's address — sbction c. 



compression. Below the lighter granitic magma should lie — 

 first, rocks of chemical composition and specific gravity 

 intermediate between those of the granites and basalts, then 

 the basalts, and, lastly, the ultrabasic rocks. 



The depth at which the basalts might first be reached 

 below the earth's surface might exceed twenty miles if the 

 observed temperature, 2200° Fahr., at which they issue from 

 volcanoes can be taken as a test, and the rate of increase 

 above quoted be applicable as a constant to such a depth. 



For convenience, the possible upward limit of the basalts 

 may be termed the isogeotherm of 2200° Fahr. When 

 heavy sedimentation takes place over any portion of the 

 earth's crust, that portion of the crust usually subsides under 

 the load. As it sinks the isogeotherms sink with it, though 

 at a less rapid rate, and so this part of the earth's crust 

 becomes temporarily extra through having its thickness 

 increased proportionately to the increase in thickness of the 

 sediments. This probably is the reason, as shown by 

 Professor Prestwich, Professor Hutton, Mr. Mellard Reade, 

 &CC. why volcanic action seldom occurs in areas of subsidence. 

 Sedimentation, however, at once introduces a local element 

 of weakness by bending the earth's crust out of its normal 

 curve, and volcanic action may occur even while subsidence 

 is still in progress along such lines of sharp fold. More often 

 the greatly increased thickness of the cold portion of the 

 earth's crust owing to the temporary depression of the 

 isogeotherms more than compensate for the weakness in the 

 crust caused by downward bulging, and the result is a 

 temporary checking of volcanic activity. 



In time, however, if the sediment be assumed to have 

 attained a thickness of about 30,000 feet, it may reach a 

 level below the earth's surface at which the temperature 

 and pressure under normal conditions may be sufficiently 

 high to cause the actual fusion of the rock, and its fusion 

 would be much accelerated by the large amount of quarry 

 water contained interstitially in the sediments. Previous to 

 the fusion of the overlying sediments the original crust, 

 formerly hard, upon which they were deposited, may be 

 rendered plastic by the rise of the isogeotherms, and then 

 this region of sedimentation at once becomes an area of 

 weakness in the earth's crust, and a fit spot upon which the 

 contracting zone of the earth's crust may expend its force. 

 This contraction will exercise a scjueezing action on the 

 hquid or potentially liquid portions of the earth's nucleus 



