264 



HISTORICAL GEOLOGY. 



debris of still older rocks, of which we know nothing. 

 Thus, we seek in vain for the absolutely oldest, the primi- 

 tive crust. As already said, no history can write its own 

 beginning. 



Character of these Bocks. We can only say, in 

 brief, that they do not differ very conspicuously from 

 metamorphic rocks of other times. They were probably 

 originally sands, clays, and limestones, much like those of 

 other times ; but, in this case, always very highly meta- 

 morphic and strongly crumpled (Fig. 166). The sands are 

 thereby changed into quartzites, the clays into schists, 



FIG. 166. Contortion of Laurentian strata. (After Logan.) 



gneisses, and even granites, and the limestones into mar- 

 bles. Along with these, however, are associated two kinds 

 of beds, which are worthy of note, viz., beds of iron-ore 

 and beds of graphite. In Canada the whole series is not 

 less than 40,000 feet thick. 



The greatest beds of iron-ore known in any strata are 

 found here. The great iron-ore beds of Sweden, of Lake 

 Superior (Fig. 167), of New Jersey, and the Iron Moun- 

 tain of Missouri, are in these rocks. 

 Recently, in southern Utah, in rock 

 of this age (or possibly later), have 

 been found the greatest iron-de- 

 posits, perhaps, in the world. The 

 strata here stand on edge, and the 



beds of iron-ore, being very hard, have been left by 

 erosion standing out as black, castellated, inaccessible 

 crags, 300 feet high, 1,000 feet long, and 500 feet thick. 

 In Canada and elsewhere graphite also occurs in immense 

 beds, sometimes pure, sometimes mixed with the rock. 



FIG. 167. 



