5o8 R. T. CEAMBERLIN AND F. P. SHEPARD 



under limb was on the point of breaking, another fold formed in 

 front of it, and so on. By continuing the compression sufficiently, 

 faulting took place. In one case (see Fig. 4, p. 498) an underturned 



fold, which started near the end 

 of the model farthest from the 

 active force, was eventually 

 changed into an overturned 

 fold, due to the advance of the 

 Fig. 15.— Broken overturned fold, overturned folds from the Other 

 The front of the fold has turned under, 1 o i „„ ij-„j 



„ , , , ^ ' end. Such a general tendency 



resembling the so-called head of a nappe. ^ ^ 



Two nappes in different stages of develop- and behavior may account for 



ment formed by compressing a single the OVerfolding being practically 

 layer of equal parts of paraffin and vas- ^^ ^^ ^^ie same direction in SOme 

 eline. 



ranges. 

 Conclusions in regard to Alpine structure which might be drawn 

 from these experiments are, in the first place, that the folds may 

 perhaps have developed in a different order of succession from that 

 commonly inferred, the upper nappes possibly having formed first. 

 Also in the place where one nappe is supposed to have completely 

 overridden another, it is perhaps most reasonable to infer that this 

 was accomplished rather more by faulting than by folding. 



DEVELOPMENT OF LACCOLITHIC INTRUSIONS IN FOLDED 

 FORMATIONS 



The term laccolith is ordinarily applied to structures in which a 

 fairly thick mass of intruding magma has arched up a dome. 

 While the floor of this structure is rarely seen, it is commonly 

 inferred to consist of more or less flat-lying sedimentary rocks, cut 

 only by the dike or dikes which represent the upward paths of the 

 igneous material. Some of the experiments suggested ways in which 

 a type of laccolith could be produced with folding occurring both 

 above and below the igneous intrusion. In one experiment a thrust 

 fault occurred on the steep side of an asymmetrical fold. This 

 thrust did not cut through the top layer. If some magma had made 

 its way up along this fault plane, it would have tended to spread out 

 beneath the top layer near the top of the anticline. Partial erosion 

 could then have revealed an apparent laccolith. This, however, 



