Epidemics. 29 



complete revolution is about 640, or the period here fixed, 

 by observation from history, as being the period or epoch 

 of an epidemic era. Some general reasons are given for 

 supposing the historical data for inferring an epidemic era 

 is 640, and the secular variations of the compass being 

 also 640 years for a complete revolution, are not mere 

 coincidences. 



This inference is concluded by a general expression of 

 belief that imponderable forces govern worlds and systems, 

 hold in integrity the existing mechanism of the earth 

 internally, and retain in integrity and perfect harmony the 

 suns and planets beyond our earth, and the plants and 

 animals living on its surface. 



But as an organizing and creative power, they have as 

 much to do with starting or originating mechanism as a fire 

 has to do with constructing a locomotive engine, though the 

 heat it supplies gives motor power to the mechanism. 



This leads to the conclusion of the treatise by an exami- 

 nation of the epochal and detrital theory of geology, which 

 is combated and doubted upon two independent bases. The 

 first is, that the chemical constituents, as found in granite, 

 are found in the sedimentary rocks, in such excess and dis- 

 proportion, that the one cannot be derived from the other. 



2ndly : That the mechanical effect of detritus must be to 

 mix stratum with stratum, and their contained organic 

 remains, especially those of small size, which are easily 

 transported by water from place to place, but the oppo- 



