88 THE PRIMARY PERIODS. 



diorites, which form important intrusive masses. Among 

 the economic minerals of the formation, the ores of iron are 

 the most important, and are generally found associated with 

 limestones." Interpreting Sir "William's technicalities, it 

 may be stated for the comprehension of the general reader 

 that this old Laurentian formation, which is of vast thick- 

 ness (some 30,000 feet or thereby), consists essentially of 

 hard and crystalline strata like the gneiss, mica-schists, 

 quartz-rocks, and marbles of the Scottish Highlands, or more, 

 perhaps, like the still harder and more granitic -looking 

 schists of the Scandinavian mountains. There are no sand- 

 stones, or shales, or limestones in the proper sense of the term. 

 All these have been converted, long ages ago, by heat, pres- 

 sure, and chemical action, into sparkling crystalline rocks ; 

 lines and layers of stratification are obscure and often alto- 

 gether obliterated ; veins and eruptive masses are frequent ; 

 and altogether the whole formation wears the aspect of a 

 vast and venerable antiquity. 



That the Laurentian system, like other stratified systems, 

 was deposited in the form of sands, gravels, clays, muds, 

 and other loose sediments, is beyond all question. Nature 

 has no other mode of procedure. What is wasted from the 

 lands is transferred to the waters ; nothing is lost. It may 

 change its form or place, but it is still in existence ; and this 

 incessant round of waste and reconstruction, as shown in a 

 former Sketch (No. 2), is the ordained order of the uni- 

 verse. What a wonderful metamorphism these primeval 

 sediments have undergone ! Not mud, nor sand, nor 

 gravel; not shale, nor sandstone, nor conglomerate; but 

 glistening slates and crystalline schists sandstones con- 

 verted into flinty quartz-rocks, and limestones into varie- 

 gated serpentines. And in the midst of all this metamor- 

 phism, the fossil organisms seem to have shared the same 

 fate ; for we cannot think of waters of deposition without 



