458 HISTORICAL GEOLOGY. 



With the more crystalline terranes correlation is extremely difficult. 

 This is owing to the absence of fossils ; to the uncertain value of the cri- 

 terion based on kinds of rocks ; and to the fact that no subdivision admits 

 of being traced to any great distance, except the kind which depends on 

 unconformity in bedding. Since this kind of unconformity is a consequence 

 of an orographic upturning, and mountain ranges have usually great length, 

 it will theoretically exist for long distances. Subdivisions based on other 

 kinds of unconformity, and on the characters of the rocks, are the most 

 common, and are necessarily of only local value. The study of a region 

 with reference to unconformity in bedding involves a complete investigation 

 of the positions of the planes of bedding, or foliation, wherever the rocks 

 are exposed to view. 



The beds of iron ore and the graphite-bearing schists of Wisconsin are 

 proved to belong to the later part of Archaean time — the Huronian ; and 

 this is probably true for the associated Archaean beds and schists, whether 

 massive, gneissic, or thin schists, and hence beds of iron ore are a great help 

 in correlation. The beds of limestones may yet be found to give aid in the 

 same direction. 



The study of the Archaean rocks has difficulties, but not so great as are 

 implied in the term "Basement Complex," sometimes used for the more 

 crystalline kinds, — an expression that sounds like a wail of despair on the 

 part of those that use it. 



3. Source of the material of later fragmental rocks. — The Archaean rocks, 

 and rocks made from them, are the main source of the material of sub- 

 sequent non-calcareous fragmental rocks. Volcanic eruptions have added 

 a little to the supply ; chemical depositions also a little ; and the siliceous 

 secretions of the lowest orders of plants and animals have contributed silica 

 to some extent ; but all these sources are small compared with those of 

 the Archaean terranes. Even the limestones have derived much of their 

 material from the same source, through the dissolving waters. The areas 

 were well distributed over the continent for supplying, through the help of 

 the ocean, mud, sand, and gravel for the deposits that were in progress as 

 the next era opened — better even than is now apparent, since many once 

 exposed are now covered, especially along the sea-borders, where the later 

 rocks have often great thickness. And their contributions have continued 

 ever since to be used in rock-making, both directly and through the strata 

 which had been made from them. 



4. The first of living species. — Science has no explanation of the origin of 

 Life. The living organism, instead of being a product of physical or 

 chemical forces, controls these forces for its higher forms, functions, 

 and purposes. Its introduction was the grandest event in the world's 

 early history. 



It is probable that the first species were of the simplest kinds ; that the 

 animals were devoid of special organs of sense, and of motion, excepting 



