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154 STRUCTURAL AND FIELD GEOLOGY 
interesting experiments by the famous French geologist, A. Daubrée, 
tend to show that two series of intersecting joints might be expected to 
result from powerful crustal movements. Daubrée experimented upon 
long rectangular plates composed of various substances, and demonstrated 
that these, when subjected to the strain of torsion, were traversed by two 
sets of approximately parallel cracks, one system crossing the other at 
angles of 70° to go”, and thus closely simulating the intersecting master- 
joints of stratified rocks. 
Mr W. O. Crosby has suggested another explanation of the normal 
intersecting joints so characteristic of bedded rocks. He thinks that 
these are probably due to earthquake action. The fractures produced 
by vibratory movements of the earth’s crust he shows must be plane, 
parallel, intersecting, and normally vertical, thus possessing all the 
characteristics of master-joints. Mr Crosby thus appeals to a vera causa, 
but his theory does not exclude that which would attribute dip- and 
strike-joints to folding. It affords a better explanation, however, of the 
vertical intersecting joints of horizontal strata, which can hardly be 
accounted for by torsion. Undisturbed horizontal strata, covering wide 
regions, are often as regularly jointed as strata which have been folded. 
In such cases, therefore, we may suppose the jointing has most probably 
resulted from the passage of earthwaves through the rocks, the alternate 
compression and tension having been sufficient to produce fissuring. 
As there is possibly no part of the earth’s crust which has not experienced 
earthquake shocks and vibrations, such crustal movements may have 
played a more important réle in the formation of joints than might be 
suspected. The great crustal movements which resulted in the buckling 
up and folding of strata in gigantic mountain chains, must often have 
induced severe earthquakes, caused by the sudden yielding of rock- 
masses to tension ; but the fissuring and shattering due to the passage 
of such vibrations or waves of elastic compression could not now be 
distinguished from the ordinary effects of folding and torsion. 
