PRINCIPLES OF STRATIGRAPHY 



CHAPTER I. 

 GENERAL INTRODUCTORY CONSIDERATIONS. 



Stratigraphy in its broadest sense may be defined as the inor- 

 ganic side of Historical Geology, or the development through the 

 successive geologic ages of the earth's rocky framework or litho- 

 spliere. Its relation to other branches of geologic science may be 

 made clear by a few brief considerations regarding the earth as a 

 whole, and a short survey of the science of geology and its subdi- 

 visions. 



THE EARTH AS A WHOLE. 



The earth as a whole may be viewed as a central mass or sphere 

 of unknown material surrounded by a number of envelopes. For 

 convenience sake in discussion, these envelopes may be considered 

 as coVitinuous shells or hollow spheres, the outer ones more or less 

 intimately interw^oven with the upper part of the rocky crust. They 

 may be grouped into the inorganic and the organic spheres. Begin- 

 ning with the outermost of the inorganic, the following are recog- 

 nized : L Atmosphere or sphere of gas and vapor; H. Hydro- 

 sphere or sphere of water; HI. Lithosphere or the rock sphere or 

 solid part of the earth. Below the known crust of the earth is the 

 zone of volcanic activities and the seat of lava formation. This is 

 called the Pyrosphere, IV, and it merges upward into the litho- 

 sphere (III) and downward into the Centro- or Barysphere, V, 

 which occupies the unknown center of the earth and the nature 

 of which is in doubt. Permeating more or less all of these spheres, 

 except the last two, is the organic or Biosphere, VI, the living en- 

 velope of the earth. A brief summary of each may first be given. 



I. THE ATMOSPHERE. This consists of a mechanical mix- 

 ture of nitrogen about 79 parts, and of oxygen about 21 parts (by 

 volume), with small quantities of carbon dioxide (about 0.03 part of 

 the whole) and some of the rarer elements. Besides this it nearly 

 always contains a variable amount of aqueous vapor together with 

 other impurities, such as dust, organic matter, etc. Its weight at 



