74 WALKS AND TALKS. 



I have included also in the foregoing table a column for 

 Groups or Periods, because some readers will be glad of this 

 more detailed information. This is a dry and unadorned 

 skeleton; but geology knows how to clothe it in flesh 

 and beauty. 



XIII. COURSES OK THE EARTH'S MASONRY. 



HOW THE FORMATIONS ARE ARRANGED. 



FIRST, let me explain what is meant by a formation. It is 

 the mass of rock resulting from some action continued uni- 

 formly to a conclusion or a pause. It was indicated in our 

 last Talk that the conditions of the world must have changed 

 from time to time, and that the nature of the ocean sediments 

 must have changed correspondingly. The sediments laid 

 down during the time in which we conclude to say no change 

 occurred, are one formation. After this, a slight change 

 would result in another formation. But these two formations 

 may much resemble each other, though decidedly different 

 from the contiguous formations above and below. These two 

 formations together may, therefore, be said to constitute a 

 formation in a larger sense, accumulated during a 'time when 

 the main action continued the same, though in subordinate 

 particulars it changed. Formation, therefore, is a general 

 term, not always signifying the same amount of accumulation 

 nor even the same range of diversity. We may employ it in 

 various applications, and we shall find it convenient to have 

 such a term. A System is a "formation;" a Great System 

 is a "formation ;" a coal-bed is a "formation ;" a river-terrace 

 is a "formation," and a metallic vein is a "formation." I 

 must state, however, that the term is not employed by all 

 geologists in this indefinite sense. 



Glancing back, now, to the beginning of sedimentary for- 

 mations, we recognize two principles which must be accepted. 

 First, the oldest or lowest sedimentary formation must have 

 rested on a foundation not sedimentary. The nature of that 

 foundation will have some light thrown upon it after we have 



