450 HISTORICAL GEOLOGY. 



chlorite schist and dioryte, and have jaspeiy bands. In 497, 499, from 

 Essex County, N.Y., the associated rock is gneiss, and the ore-bed is 

 interhiminated with quartz. At one Essex County mine, tlie ore-bed is 150 

 feet thick ; at the Cranberry mine, on the borders of North Carolina and 

 Tennessee, 400 feet. Grains of calcium phosphate (apatite) are often 

 disseminated through the ore. 



Iron carbonate is associated with the oxides south of Lake Superior. It 

 occurs only sparingly to the eastward in Michigan, south of Lake Superior, 

 at the Marquette mine, but more abundantly to the westward in Wisconsin. 

 The metamorphism of the beds, correspondingly, is least to the westward. 

 The carbonate is the ore originally laid down, and the hematite and magne- 

 tite are results of metamorphic change, in which the carbonic acid was ex- 

 pelled. 



In eastern Canada and along the Archsean protaxis, southward through New York, 

 New Jersey, and beyond, the carbonate is wholly absent, the iron ores being magnetite, 

 hematite, or titanic iron. Moreover, the thickness of the ore-beds is far greater and the 

 metamorphism of the region is of higher grade, — thick-bedded, massive, and schistose, 

 crystalline rocks prevailing. Notwithstanding these differences, the eastern iron-bearing 

 series may he Huronian, and unconformable to adjoining Laurentian, but the evidence of 

 this has not been obtained. The same belts have their thick beds of crystalline limestone, 

 often chondroditic, and in this respect rocks of the Appalachian protaxis differ from those 

 of the Lake Superior region. The course of the Appalachian chain was the region in 

 later time of thick sedimentary deposits, great upturnings, intense metamorphism, while, 

 cotemporaneously, little change was in progress over the Mississippi Valley ; and it may 

 be that the same kind of difference distinguished the two regions in Archsean time. 



STRUCTURE, THICKNESS, AND ORIGIN OF THE ROCKS. 



As is implied in the preceding descriptions, part of the rocks are massive^ 

 as granite, syenyte, dioryte, gabbro ; and a large part are schistose and dis- 

 tinctly stratified ; and into the schistose the massive often graduate. The 

 alternations of ore-beds with schists, quartzyte, limestones, in sections like 



those figured above, are evidence of strati- 

 fication, and, therefore, of the succes- 

 sive formation of the beds, whether now 

 crystalline or not. The quartzytes are old 

 ^ a sandstones; the limestones deposited beds 



Interstratified limestone, St. Lawrence ^f limCStone, either of Organic Or chcmical 

 County, N.Y. Emmons. . . 



origin ; and the schists are iragmental beds 

 in a metamorphic condition. In Fig. 500 a stratum of limestone, I, is overlaid 

 by strata of gneiss, a, a, and steatyte, b. Such sections could be multiplied 

 indefinitely. The following, by Logan, Fig. 501, which is partly ideal, but 

 not untrue, represents white granular or crystalline limestone, a, many 

 times folded and interstratified with gneiss and quartz rock, b ; and the 

 limestone has been traced over the same region (Grenville and the adjacent 

 country, Canada), in the linear and curving bands of a series of great 



