504 



R. T. CHAMBERLIN AND F. P. SHEPARD 



facilitated the adjustments necessary in this type of folding, and 

 thus constituted zones of accommodation. 



In several cases a plastic layer between two competent layers 

 pinched out completely for a short distance. This is comparable 

 to some of the occurrences in the Alps where a layer has been so much 

 thinned that it has completely disappeared from the section. Stran- 

 gle folds which are also found in the Alps, and in a few other local- 

 ities, were reproduced in the models (Fig. 9). In some cases the 



Fig. 10. — Same model as Fig. 9 after 12 inches of compression. The upper com- 

 petent layer has been puUed apart by upward flowage of the plastic material beneath. 



Fig. II. — Layer 2 made of plaster; other layers 3 paraffin, i vaseline; 23 inches 

 compressed to 14I inches in 30 minutes. Fracturing of the strong layer (No. 2) 

 caused considerable fracturing above and below. Open spaces represented by black. 

 Note the slight degree of folding in the bottom layer. 



competent layers were faulted, so that there were two disconnected 

 ends, but the fault was largely obscured because the plastic material 

 flowed in. 



Rigid layer in the midst of plastic material. — Where a layer of 

 plaster of Paris was inserted between moderately plastic layers some- 

 what different results occurred. Since the plaster of Paris was too 

 brittle to be folded, it invariably broke. In breaking it caused con- 

 siderable fracturing of the surrounding layers (Fig. 11). Conditions 

 of this sort may account for an unexpected amount of fracturing in 

 some types of sedimentary rocks which would normally be more 



