26 R. T. CHAMBERLIN AND W.. Z. MILLER 
Several strong layers between very much weaker ones.—When 
there were several strong layers between much weaker ones, similar 
arching was sometimes the result, if there was sufficient difference 
in the rigidity of the layers. But rather more commonly a strong 
tendency to dovetail was manifested. The strong and brittle 
beds of plaster were first thrust faulted, but in some cases the 
planes of faulting dipped toward the moving pressure block and in 
other cases away from it, thus giving both underthrusting and 
overthrusting (Fig. 9). After faulting commenced, the broken ends 
of the strong layers were wedged into the adjacent softer material, 
pushing it aside. Further compression caused an interwedging of 
PRESSURE 
BLOCK 
RESISTING F 
BLOCK F 
Fic. 9.—Development of dovetailing structure by both overthrusting and under- 
thrusting when there is too much difference in the competency of the layers. With 
further compression the dovetailing becomes more pronounced. ‘The strong layers 
were composed of four parts of clay and one of plaster; the weak layers were of 
damp sand. 
the competent beds, producing a dovetailing structure. Dove- 
tailing therefore results when there are a number of strong layers 
and the layers are too unequal in competency. This is commonly 
the case when there is an alternation of plaster layers and layers 
of damp sand. It even takes place when the competency of the 
strong layers is reduced by adding four parts of clay to one of 
plaster. This seems to be because the sand is very incoherent. 
When dovetailing does occur in experimentation, obviously not 
much is to be learned from the results so far as the present problem 
is concerned. 
Less difference in competency.—The conditions of the last set of 
experiments apparently do not at all approach the conditions which 
control faulting in the earth. There was too great difference in the 
relative strength of the layers. The weak layers were too weak. 
For the next set of experiments sand was abandoned and only 
