150 GEOLOGY. 



freights. In such cases it is very easy to find the source 

 of the boulders. Drift, in its passage, left in many places 

 its marks, as scratches, and furrows, and smoothings. 

 Sides of mountains exhibit often these marks, and so 

 high up in this country that we know that there is only 

 one mountain in New England, Mount Washington, that 

 lifted its head above this drift action. I will not dwell 

 longer on this interesting subject here, as it will be 

 brought before you more particularly in another part of 

 this book. 



233. Subsidences and Elevations of the Earth's Crust. 

 You will find, as I proceed to notice the different ages 

 of the earth's formation, that different portions of its 

 crust were alternately submerged under the waters and 

 elevated above them. This was not done by any sud- 

 den convulsive movements, but slowly, perhaps as slowly 

 as the change of level now occurring in Sweden and 

 some other countries, as noticed in 206. These altern- 

 ate subsidences and elevations, which were generally 

 several, sometimes many in number, were necessary, as 

 you will see, for the formation and arrangement of the 

 various strata that make up the earth's crust. Some- 

 times subsidences have been prolonged through even 

 many ages. Lyell speaks of one over a large area in 

 England and Wales which continued so long that strata, 

 over six miles (32,000 feet) in thickness were deposited 

 during the time. In all this period, covering not an age 

 merely, but successive ages, there was an ocean's bed in 

 that region, and this bed sank gradually and quietly as 

 the deposits of solid matter were made upon it from the 

 water lying above it. In no one of the ages of the 

 earth's formation was there so remarkable an alterna- 

 tion of subsidences and elevations as in that one in which 

 the coal was made and laid down. Each bed of coal, as 

 you will learn more fully in another chapter, was made 

 from vegetation grown upon the surface, and this being 

 submerged by a subsidence, rock was formed over it. 



