BLOCK MOUNTAINS 417 



old, they would long ago have been worn down. The older 

 mountains of the earth are all comparatively low, not 

 because they were never elevated as high as the lofty 

 mountains of to-day, but because their greater age has 

 longer subjected them to erosion and thus reduced their 

 height. 



It is difficult to classify the different kinds of mountains, 

 for very few of them are simple in their structure, but 

 certain kinds of mountain forms are easily distinguished. 



196. Block Mountains. Experiment 131. Take three pieces 

 of smooth, straight-edged boards, two of which are about 15x55 cm. 

 on a side and the other 8 x 55. Place these flat on a table with the 

 smaller board in the middle and the longer edges close together. 

 Sift corn meal, fine coal dust, powdered pumice, plaster of Paris and 

 fine sawdust in even layers over the boards. Now lift carefully the 

 inner edge of one of the wider boards and slip under it a narrow strip 

 of wood 1 or 2 cm. thick. The layers of material spread over the 

 boards will be broken and slant back from the line of breakage with 

 their edges exposed along this line. Do the same with the wide 

 board on the other side. The conditions shown will be similar to 

 those exhibited in block mountains. 



In southern Oregon and extending southward are found 

 long, narrow mountain ridges, having a steep cliff on one 

 side and a gentle slope on the other. Between these 

 ridges are flat, troughlike depressions in which Small lakes 

 are sometimes found. The ridges are formed of thick 

 layers of rock inclined at the same angle as the long slope 

 of the ridge. The short slope of the ridge exposes the 

 edges of these layers which have been broken across. 



The debris slopes at the foot of the steep cliffs in some 

 cases are slightly broken across in a direction parallel to 

 the cliff. The steep cliffs sometimes face each other with 

 a somewhat flat depression between, and sometimes the 

 cliff on one ridge faces the long slope of the next. Some 



