Time-table for North America. 19 



Chamberlin.* As the names of successive great divisions of 

 earth history, to be applied in widely separated regions, these 

 terms Archeozoic and Proterozoic imply a less definite correla- 

 tion than the more localized terms of Paleolaurentian, 

 Neolaurentian, and Algonkian, used here for the Canadian 

 Shield. For this reason these "zbic" names appear to have 

 real value, as well as for the fact that by their use harmony is 

 maintained through the whole scheme of geologic chronology. 

 The nature of the faunas of the Proterozoic and Archeozoic is 

 unknown, as is also the time in earth history when the Metazoa 

 first rose to dominance over the Protozoa. The dividing line 

 therefore cannot be drawn from biologic evidence; but even 

 if a fair knowledge of the life of these times was possessed, it 

 is probable that it would be found gradational to a considerable 

 degree, and these broad names as here used could still apply 

 without doing violence to the biotas of the Paleo- and 

 Neolaurentian. 



In this table the name Huronian has been restricted to the 

 series originally studied by Logan. The Animikie, often called 

 the Upper Huronian, is separated by a wide-spread unconformity 

 and in its wider regional extent is distinct from the original 

 Huronian. On the other hand, many of the areas formerly 

 called Lower Huronian are composed of rocks which are sepa- 

 rated by a crustal revolution and a following great erosion inter- 

 val from the true Huronian. Three distinct series have thus 

 become linked in past decades under one name, but the tendency 

 of modern classification is in just the opposite direction. That 

 which is here still called Huronian is no doubt susceptible 

 of division, but there would be questionable value in introduc- 

 ing such subdivisions in this table and it would involve correla- 

 tions which only those personally familiar with the fields 

 should undertake. 



The first great advance in the understanding of the Lauren- 

 tian, the basal Archean, lay in the recognition that the gneisses 

 were largely of igneous origin and were younger than certain 

 lavas and sediments which rested upon them. It became 

 apparent that vast domes and irregular bodies of molten rock 

 had welled up from the unknown depths, had displaced and 

 engulfed the older foundations of the crust, and had permeated 

 and altered the surficial rock cover which still remained. 

 These great masses of igneous rock are . known as batholiths. 

 In the invasion and injection of the older rocks they have 

 absorbed material into themselves and added their emanations 

 to the enveloping rocks. Mountain-making pressures also 

 came into play and combined their action with that of igneous 



*Van Hise and Leith, Pre- Cambrian Geology of North America, Bull. 

 360, U. S. Geol. Surv., 1909, p. 21. 



