SOME EXPERIMENTS IN FOLDING 



505 



folded. In one case an anticline developed above the plaster layer 

 and a somewhat fractured syncline below. This behavior was 

 apparently due to a thickening of the intervening plaster by slice 

 faulting. The whole mass was so much broken that it was difficult 

 to trace the individual planes. Such an occurrence might easily 

 cause confusion in an oil field by changing the surface anticline into 

 a syncline below. In general, where an especially brittle formation 

 occurs between more easily folded formations, structural changes in 

 depth are very likely to occur. 



ISLANDS OF RIGID MATERIAL 



Sedimentation has frequently occurred in seas where islands or 

 submerged reefs of metamorphic or igneous rock have stood above 

 the general level of the sea bottom in such a way that the sediments 



:^ 



Fig. 12. — Plaster island in midst of weaker beds. Bottom layer 2 parafiSn, i 

 vaseline; middle layer, i par., i vas., i plaster; top layer, 4 par., i vas. 



East side of model (middle figure) : Beds nearest pressure block turned under and 

 slid upward over the rigid plaster. 



West side of model (lower figure) : Beds on both sides turned imder and sliding 

 upward carried some of the plaster with them. 



were deposited all round them, and in many cases eventually cov- 

 ered them. In order to get some idea of what might be the effect 

 of such a buried crystalline mass upon folding which subsequently 

 affected the region, compression experiments were tried upon a model 

 which consisted of a mass of cement with outward sloping sides, 

 placed within a succession of plastic layers (Fig. 12). Compression 



