FOLDED MOUNTAINS 419 



Taking hold of the strip, stretch the rubber dam as much as it will 

 readily stand. Fasten the strip so as to hold the rubber dam in this 

 stretched position. 



Sift fine sawdust, plaster of Paris, fine coal dust, ground pumice, 

 corn meal, or any other distinctly colored substances in even layers 

 over the stretched rubber dam. Slightly dampen the layers. Re- 

 leasing the strip, allow the rubber dam to contract very slowly. When 

 it has fully contracted, cut carefully through the layers of material 

 with a thin knife and remove that which is on one side of the cut. 

 The layers will have been folded into irregular undulating folds, thus 

 simulating folded mountains. 



Where layers of rock are subjected to slow, uniform 

 and tremendous lateral pressure, they may form undulat- 



Fig. 119. 



ing folds with little fracturing. (Fig. 119.) The contract- 

 ing of the interior of the earth, due to cooling, has some- 

 times brought to bear such pressure, and in a few cases 

 undulating folds have been produced. 



The best example of this folding is that of the Jura 

 Mountains, between Switzerland and France. Here the 

 almost regular folding of the strata can be seen wherever 

 the streams have cut across the mountains. The moun- 

 tains are so young that there has been little carving by 

 erosion and the downfolds still form the valleys and the 

 upfolds the ridges. 



The rock layers composing these folds contain marine 

 fossils, showing that they were once horizontal and must 

 have been formed in the sea. The longer streams run 

 down the troughs of the folds, but in some places, often 



