T. STERRY nr\T. 41 -Hi I 



iron-ore which to-day constitute the chief mineral wealth 

 of the Laurentian series of our region. These, and sim 

 ilar rocks, so far as we know, formed everywhere the 

 dry land and the ocean's bottom. At that time the crys- 

 talline schists which form the greater part of New 

 England and Eastern Canada ; the slates, limestones and 

 iron-ores of Dutchess county and the great Appalachain 

 valley were not yet formed ; the vast series of paleozoic 

 rocks, some 40,000 feet in thickness which includes the 

 salt and gypsum and iron-ores of central New York, 

 and the coal and petroleum of Pennsylvania, had not 

 been laid down. The precious stores of lead and silver 

 ores in Colorado, Utah and Nevada, the mineral wealth 

 of the Sierras, were not ; for the very rocky strata in which 

 all these deposits are included were as yet unformed. 

 All of the great geological series just enumerated have 

 been built up since the Laurentian age. and in great part 

 from the ruins of portions of the rocks previously ex- 

 isting, and owe, with limited exceptions, their deposition 

 and their accumulation to the action of water. The 

 changes and revolutions of the earth's surface during these 

 long ages are written on the rocky strata, and the remains 

 of organisms which these enclose, tell the history of the 

 successive generations of plants and animals which have 

 flourished and passed away. Time would fail us to con- 

 sider the differences in texture and in composition be- 

 tween these later formed rocks and the more crystalline 

 strata of Eozoic time, or to speak of the great masses of 

 unstratified crystalline rocks, such as the granites of 

 New England and the diabases of the Palisades, both of 

 which are evidently posterior in origin to the stratified 

 rocks among which they have been intruded in a heated 

 and more or less molten condition, not unlike the volcan- 

 ic rocks of more recent times. The interstratified masses 

 of aqueous origin are often distinguished by the obvious 

 name of indigenous rocks, -while the intruded masses 

 are sometimes spoken of as exotic rocks. These, from 

 their resemblances in composition to the older stratified 

 rocks, are conjectured to be no other than portions of the 

 latter which have been softened by heat in the depths o\' 

 the earth, and in this state forced upward among the 

 overlying rocks. 



