LAURENTIAN SYSTEM OF ROCKS AND ARCHJ3AN ERA. 285 



fore, be quite general, and will not enter into any subdivisions, al- 

 though this era was very long. In the next era we will go into the de- 

 scription of the several ages, in the next into the periods, and in the 

 next even into the epochs. 



Prehistoric Eras. — Previous to even the dimmest and most imper- 

 fect records of the history of the earth there is, as already said (p. 278), 

 an infinite abyss of the unrecorded. This, however, hardly belongs 

 strictly to geology, but rather to cosmic philosophy. We approach it 

 not by written records, but by means of more or less probable general 

 scientific reasoning. We pass on, therefore, without pause to the low- 

 est system of rocks containing the record of the earliest era. 



CHAPTER II. 

 LAURENTIAN SYSTEM OF ROCKS AND ARCHAEAN ERA. 



It is one of the chief glories of American geology to have estab- 

 lished this as a distinct system of rocks and a distinct era. 



It had been long known that beneath the lowest Palaeozoic rocks 

 there still existed strata of unknown thickness, highly metamorphic 

 and apparently destitute of fossils. These had been usually regarded 

 as lowermost Palaeozoic — as the earliest defaced leaves of the Palaeo- 

 zoic volume. But the study of the Canadian rocks, by Sir William 

 Logan, revealed the existence of an enormous thickness of highly-con- 

 torted, metamorphic strata, everywhere unconformable tvith the overly- 

 ing Primordial or lowest Silurian. More recent observations show this 

 relation not only in Canada, but also in New York, on Lake Superior, 

 in Nebraska, Montana, Idaho, Wyoming, Colorado, Utah, Nevada, 

 Texas, New Mexico, and Arizona. Nor is it confined to our own coun- 

 try, for the same unconformable relation has been found by Murchison 

 on the west coast of Scotland, between the lowest Silurian (Cambrian) 

 and an underlying gneiss, evidently corresponding to the Laurentian 

 of Canada. Similar rocks, and in similar unconformable relation, have 

 been found underlying the primordial. in Bohemia, and also in Sweden 

 and Bavaria, and many other places. Such general unconformity 

 shows great and wide-spread changes of physical geography at this 

 time, and therefore marks a primary division of time. There seems 

 no longer any doubt, therefore, that it should be regarded as a distinct 

 system. 



The following figures (256, 257, 258) give the relation between the 

 Palaeozoic and the Laurentian in New Mexico, in Canada, and in 

 Scotland. 



