MICHIGAN. 



HISTORIOzVL GEOLOGY OF MEOHIGAlSr, 



AS INDICATING THE ORIGINAL SOURCES AND PRESENT CONSTITUTION 



OF ITS SOILS. 



The actual causes of the present condition of the earth's surface lie hid- 

 den among the events of ages long antedating the pages of history and the 

 existence of man. 



The deductions of the geologist, therefore, necessarily take the form of 

 theories, usually based upon the assumption that the earth's mass was once in 

 an incandescent and fluid state ; and that, by the process of general cooling, 

 and the subsequent disposition and influence of moisture and frost, acting 

 through lengthened periods, it has gradually reached its present condition. 



These theories also proceed upon the assumption that, as soon as the sur- 

 face became habitable, it was occupied by plants and animals adapted to 

 existing conditions ; which, from time to time, as conditions changed, gave 

 place to new types of existence, leaving behind them, however, the evidence 

 of such existence in the coal beds and in the petrifactions occurring in the 

 rocks of those periods. 



A careful and extended study of these affords the means of determining 

 the relative ages of the strata in which they occur ; while the relative ages of 

 the strata of aqueous origin are further indicated or inferred from the 

 natural order of their deposition from water. 



The varying theories devised by prominent geologists to account for exist- 

 ing facts have long since passed the ordeal of discussion and collation, and 

 have become digested into something akin to a formula; which has come to 

 be generally accepted. Such formula supplies the basis upon which to con- 

 struct what is designated as the historical geology of a region. 



