GEOLOGY 



CHAPTER I. 

 THE ORIGIN OF THE EARTH. 



The strata of the earth reveal its history with great fidelity for 

 long periods previous to the present, but earlier than that the record 

 becomes indistinct, and if we attempt to follow it back to the beginning, 

 the indistinctness merges into extreme obscurity. The rocks at the 

 base of the known sedimentary series are so greatly disrupted, crumpled, 

 crushed, metamorphosed, and traversed by intrusions, that their his- 

 tory is deciphered with the greatest difficulty and no little uncertainty, 

 while below these lies the inaccessible interior of the earth whose for- 

 mation constituted a still earlier chapter in the history. The nature 

 of this inaccessible mass can only be inferred from volcanic extru- 

 sions, the transmission of seismic tremors, the phenomena of gravity, 

 the distribution of rigidity and of internal heat, the modes and pro- 

 cesses of deformation, and other phenomena of a more or less dynamic 

 kind. All these phenomena have their bearing on the problem of 

 the earth's origin, but just what they imply cannot yet be interpreted 

 without some measure of reasonable doubt. 



Besides these internal phenomena, suggestions relative to the 

 origin of the earth are to be found in its characteristics as a planet, 

 and in its relations to the other members of the solar system. Sug- 

 gestions are also to be found in certain features of the solar system 

 which show that it had no haphazard origin. The birth of the system 

 is beyond doubt revealed in its constitution and in its dynamics, if 

 one could but read the record. But all these phenomena of external 

 relations, as of the hidden interior, are difficult to interpret, and 

 the meaning they carry cannot be read as we read the sedimentary 

 record. We do read dynamical records. In the fall of rain we readily 

 read the previous ascent of vapor, not so much by any material record 



