V. THE MODE OF OCCUREENCE OF ROCKS 



It is ordinarily assumed that the earth owes its present form 

 to having originated from a mass of incandescent vapor, and 

 passed, by gradual cooling and consequent condensation, from 

 gaseous through pasty or fluidal, and all intermediate stages 

 to its present condition. This, in brief, is the hypothesis of 

 Kant, and seems most readily to account for the facts as we 

 now know them. As to the character of the rock masses result- 

 ing from this primary cooling, little is known. Reasoning from 

 analogy, it seems safe to assume that they resembled the slags 

 from a smelting furnace, or some form of modern lavas, more 

 nearly than any other rock masses of which we have knowledge. 

 "Whatever may have been their nature, they have long since 

 been obscured by rocks of secondary origin, or become so altered 

 through dynamic and incidental chemical agencies as to be no 

 longer recognizable. 



The oldest rocks of which we now have knowledge belong 

 to the group of gneisses and crystalline schists. They are as 

 a rule highly siliceous rocks, though frequently including con- 

 siderable thicknesses of crystalline limestone. They contain no 

 traces of what can be referred beyond doubt to an organic origin, 

 but from their banded or foliated structure, so closely simulating 

 bedding, they have in the past been considered as metamorphic ; 

 that is, as rocks laid down as sediments and crystallized by the 

 complex processes comprehended under the term metamorphism. 

 Rocks of this type, according to Dana, first appeared in North 

 America in the wide Y-shaped area extending from Labrador 

 southwesterly to the Great Lakes, and thence northwesterly to 

 the Arctic regions. This area has since been added to by the 

 folding and crumbling processes incident to the formation of 

 the Appalachian and Rocky Mountain systems. Concerning the 

 geographical distribution of these rocks, as they now appear 

 exposed, nothing need be said here. They seem to form, as 

 has been stated, the actual floor of the continents upon which 

 all later deposits have been laid down, and through which and 



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