Shaler.] 292 [February 3, 



there is a considerable variation, amounting often to nearly half the 

 annual change between the temperatures of day and night. Tliis 

 incessant change in the temperature of the outer portions of the mass 

 must be accompanied by a considerable movement, or at least a ten- 

 dency to move, of the portion afi'ected by the changes of temjierature. 

 The change of temperature of 150 degrees Fahr. in a sheet of granite 

 one hundred feet in diameter, would produce a lateral expansion of 

 about one inch at the surface. The amount of the change in temper- 

 ature would gradually diminish from the surface towards the interior. 

 We are unfortunately without any accurate data which would enable 

 us to determine the depth to which changes of temperature affect the 

 heat of massive rocks. The data derived from observations which 

 have been made on soils are so far compUcated by the effect of perco- 

 lating water as to be useless. It is, however, quite plain that at the 

 depth of a very few feet we shall discover material which is essentially 

 unaffected by any variations of temperature. It is also quite easily 

 seen that this outer region of rock, which is in almost constant move- 

 ment from the changes of temperature, must tend to detach itself 

 fi'om the immobile mass. If we consider the effect of heating the 

 surface of a table of rock, it will become apparent that the part ex- 

 posed to the sunshine tending to expand while the unheated jjortion 

 remains unaffected, there will arise a movement which, unless over- 

 come by the cohesion of the materials, will separate the outer portion 

 of the rock from the interior mass. The tendency will be to form a 

 shell upon the centre of the mass, elevated from it except upon the 

 edges. 



In many cases there would probably be two lines of separation 

 formed, one at the limit of annual variation, and another at or near 

 the point where the extreme daily variations cease to be felt. 



It has frequently been noticed in regions where massive rocks 

 abound, that certain points give forth a sonorous sound when sti-uck 

 by the foot, — a sound which suggests to the ear that there is a cavity 

 beneath the surface at that point. A tolerably careful examination 

 of one or two such points has satisfied me that the resonance is due 

 to the fact that the heat of the sun has caused some of the larger 

 sheets of the concentric layer to arch upwards so that it rests upon its 

 edges, not being in contact with the subjacent mass in the centre, jthus 

 making a very shallow cavity, which, however, is large enough to give 

 a resonance to the upper wall. This resonance seems to be greater 

 after long continued hot weather, as would be expected if this hy- 

 pothesis were true. 



