256 Dr Hrszert on the Limestone of Burdiehouse 
This varying, yet tranquil, alternation of sandstone, argilla- 
ceous shale, coal, and limestone, some of which are of fresh-water, 
and others of marine origin, becomes of difficult explanation. No- 
thing is more susceptible of proof, than that, during the carboni- 
ferous epoch, the crust of the globe was subjected to continued 
and prolonged oscillations of movement ; and these indicate an 
ancient instability affecting our planet, of which, in reference to 
the comparative state of unchangeableness which now prevails, 
we can have little conception. 
These circumstances of alternation have recently engaged 
much attention.—It has been shewn, that, during the secular re- 
frigeration to which the globe is subject, the fractured portions 
of its crust would continue to adapt themselves to the diminish- 
ing capacity of its nucleus, in order to embrace it more closely ; 
but it may be now explained, that, during this process, the same 
fractured portions would have their action modified by other cir- 
cumstances, namely, unequal degrees of contraction, caused by 
thermometrical differences beneath the earth’s surface. These I 
shall attempt to explain. 
A theory to this effect, proposed by Mr Dr La Becne and 
Mr Bassacs, refers to the heated state of the nucleus of the 
globe, and its power of radiating heat. It also assumes, that the 
surfaces beneath rocks possess different powers of radiating heat. 
It follows, then, that the numerous fractured portions into which 
the crust of the earth has, by convulsions, been resolved, would, 
in reference to subjacent thermometrical differences, manifest 
unequal contractions ;—that occasionally these unequal contrac- 
tions would cause the sides of great fissures to press against each 
other, so as to induce sudden dislocations and movements ;—but 
that more frequently very slow movements would be induced, 
too slow, perhaps, to be measured by time, attended with new po- 
sitions given to great masses of rocks or strata. Large areas 
would be gradually thrust upwards along their lines of fracture, 
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