SOME EXPERIMENTS IN FOLDING 509 



would probably be of only modest dimensions, and might be classi- 

 fied as a phacolite/ 



In another case a wedge fault occurred in the lower layer on the 

 limb of an antichne. Since this wedge faulting took up some of the 

 shortening of this layer, it was not arched as high as the upper layers 

 and thus left a cavity between. In nature, a potential cavity might 

 be produced similarly, and magma, making its way up along the 

 wedge fault, and between the layers, might help force the produc- 

 tion of the cavity and fill it. Such an occurrence could also pro- 

 duce a structure which had all the surface appearance of a com- 

 mon laccolith. 



Rupture by tension on the crest of an anticline sometimes was 

 confined to the lower layers of a model. In such cases cracks 

 developed right across the individual members in the lower portion 

 of the model. Along these cracks an igneous intrusion could force 

 its way up as far as the unbroken layer and spread out below that 

 layer. 



INCIDENTAL OBSERVATIONS ON FAULTS 



Dying out of faults. — In nature faults are known to die out 

 upward, downward, and laterally, but it is often impossible to see 

 what happens in the space between where there is a distinct fault 

 and where there is no fault. In the experiments faults of small 

 displacement frequently played out both vertically and horizon- 

 tally. In some cases the scarp of an overthrust became lower and 

 lower transversely across the model till it disappeared in favor of 

 the unbroken layer. Still more frequently the layers below an anti- 

 cline were found to be faulted while the top layer was not affected. 

 In another case the top layers were faulted and the bottom layer 

 was merely folded (Fig. 16, p. 510). As sections could be cut into the 

 models at any place desired, it was not difficult to observe the man- 

 ner in which these faults died out. In general the most common 

 manner was by a thickening and thinning of the layers in such a way 

 as to take up the waste space. A slight local bending after the fash- 

 ion of a minor monoclinal flexure in many cases facilitated the accom- 

 modation. Another method was by the formation of a cavity at 

 the top of the fault on the downthrow side. 



^ Alfred Harker, Natural History of Igneous Rocks, 1909, pp. 77-78. 



