142 



GEOLOGY. 



cipal modes. I have already mentioned the agency of 

 flexure or plication, as illustrated by Fig. 52. Denuda- 

 tion has much to do with the formation of such mount- 

 ains, as seen in Fig. 69. Here you see on each side 



Fig. 69. 



strata which, if continued on over the mountain, would 

 very much increase its height ; but they were removed 

 probably by the water sweeping over the mountain all 

 the time that it was slowly rising. This is a good rep- 

 resentation of the manner in which most of the mount- 

 ain chains are made and shaped. 



Take another ex- 

 ample, represented 

 in Fig. 70. Here it 

 is obvious that the 

 strata a were first 

 deposited horizon- 

 tally ; then the middle part or axis of the mountain was 

 thrust up, tilting the strata a, and after the tilting was 

 completed the strata b were deposited. In Fig. 71 we 



Fig. TO. 



Fig. 71. 



see a somewhat different state of things. The strata a 

 and b were both deposited before the axis of the mount- 

 ain rose, and therefore were tilted up together. After- 

 ward the strata c were formed. Now if the strata a 

 and b are the same in character in the two cases, the 

 mountain represented by Fig. 71 is of much later date in 

 the world's growth than that represented in Fig. 70. 



