418 7F. 0. Croshy — Absence of Joint- Structure at Great Depths. 



contraction which the rock experiences during its consolidation and 

 burial under newer formations is in the horizontal direction, in which 

 alone it can result in the formation of joints. 



But does this lateral contraction actually occur ? This seems 

 at least doubtful ; for the increase in temperature, which for a 

 reasonable depth — say 10,000 feet — must be considerable — probably 

 200° F., means a tendency to expansion. The contraction of the 

 rocks resulting in the formation of joints is, of course, measured by 

 the volume of the joint-cracks, when they have not been widened 

 by atmospheric action, etc., and are really contraction and not 

 movement joints. Now the volume of the joint-cracks will not 

 probably in ordinary rocks amount to more than a small fraction 

 of one per cent, of the volume of the rock. But, according to 

 Colonel Totten's experiments, the moderate increase of temperature 

 supposed above would produce an expansion of about '001 in marble, 

 and "002 in sandstone ; enough, probably, to close up all the joints 

 in each of these rocks, i.e. to counteract the tendency to lateral 

 contraction. 



At greater depths than 10,000 feet the influence of heat in opposing 

 tbe contraction of the rocks would be still more marked, for while 

 the temperature and pressure both probably increase at a sensibly 

 uniform rate for at least ten or twenty miles below the surface, and 

 tbe rate of expansion may be assumed as sensibly the same for a 

 range of temperature of 1000° or more, yet the tendency of the 

 rocks to contract through either compression or loss of water must 

 diminish rapidly. 



According to the argument here presented, then, it is possible that 

 sediments may be carried down to any depth in the earth's crust, 

 and attain any degree of consolidation, without becoming jointed. 

 It will be claimed (and admitted), however, that a net contraction 

 must take place sooner or later ; for this is I'equired by the universal 

 jointing of tbe rocks at the surface. But 'when and where will 

 this occur. 



Suppose that our sea-floor becomes dry land — is changed from an 

 area of deposition to one of erosion, then during the lapse of ages 

 tbe particular stratum which we are following is gradually brought 

 back toward the surface. The isogeotherms fall and its temperature 

 is lowered, and at the same time tbe pressure of the superincumbent 

 beds is removed. The cooling necessarily implies contraction ; but 

 the removal of pressure is not necessarily followed by expansion, 

 because the sediments, while subjected to pressure, and largely as a 

 result of the pressure, have become consolidated ; and if they are 

 compared to a compressed spring, it should be to a spring which has 

 received a permanent set. 



In other words, as the deeply buried stratum gradually nears the 

 surface, through erosion, it for the first time actually experiences 

 contraction in all. directions and becomes jointed ; and this is tlie 

 direct result, not of pressure or desiccation, but of cooling. The 

 cooling, contraction and jointing are proportional to, and keep pnce 

 with, the erosion. Hence it follows from this way of regarding the 



