286 



LAURENTIAN SYSTEM OF ROCKS 



These, then, are the oldest known rocks. They form the first vol- 

 ume of the recorded history of the earth. Yet they evidently are not 



Fig. 256.— Section across Santarita Mountain, New Mexico: c. Carboniferous; 8, Silurian; A, 

 Archaean; m, metalliferous vein (after Gilbert). 



the absolute oldest ; evidently they do not constitute any part of the 

 primitive crust. For they are themselves stratified or fragmented 



1 2 



Fig. 257. — Section showing- Primordial unconformable on the Archaean: 1, Archaean or Laurentian; 

 2, Primordial or Lowest Silurian (after Logan). 



rocks, and therefore formed from the debris of other rocks still older 

 than themselves ; and these last possibly from still older rocks. Thus, 



Fig. 258. — Diagram Section, showing the Structure of the North Highlands: a, Laurentian; b, Pri- 

 mordial; c, Lower Silurian (Jukes). 



we search in vain for the so-called primary rocks of the original crust. 

 Thus is it with all history. No history is able to write its oiun begin- 

 ning. 



Rocks. — There is nothing very characteristic in the rocks of the 

 Laurentian system, except their extreme and universal metamorphism. 

 They do not diifer very conspicuously from metamorphic rocks of other 

 periods ; consisting probably of altered sandstones, limestones, and 

 clays. They are all, however, very much contorted and very highly 

 metamorphic. In Canada they consist mainly of the schist series, 

 passing on the one hand into gneiss and granite, and on the other into 

 hornblendic gneiss, syenites, and diorites ; of sandstones, passing into 

 quartzites ; and of limestones, passing into marbles, which are some- 

 times even intrusive. These together, in Canada, form a series of rocks 

 at least 50,000 feet thick. 



Interstratified with these are found immense beds of iron-ore 100 or 

 more feet thick, and great quantities of graphite, sometimes impreg- 

 nating the rocks, and sometimes in pure seams. In rocks of this age 

 occur the great iron-beds of Missouri, of New Jersey, of Lake Superior, 

 and of Sweden ; and probably the mountains of iron recently found in 



