XXIV PREFACE. 



The data are much more numerous and are in need of definition and classification. 

 From our study we have been able, as in the case of the Mesabi iron ore, only to 

 reach a warrantable conclusion. Such conclusion must yet be pursued to its legiti- 

 mate results, not alone theoretically, but through an inspection of other positive data 

 and the interpretation and application of many minor steps and collateral issues. 



Unfortunately, however, these and other questions must, for the present be left 

 unsolved. It is the nature of all geological research to continually open up new 

 fields. These, however inviting, must, in the case of the present survey, be relin- 

 quished with only a mention of their existence, in order that one or two, which have 

 been sufficiently explored, may be described and thus preserved within the scope of 

 positive geology. 



The reader will find in this volume some conclusions as to the geology of the 

 Archean which, while not entirely new, having been proposed in the main by others, 

 sometimes on imperfect data and at other times with only partial apprehension of 

 their scope, yet are not everywhere accepted as valid tenets of Archean geology. It 

 may be well to succinctly mention some of these. 



1. The Archean began with the crust of the earth a basic rock greenstone. 

 This crust, on cooling sufficiently to allow the condensation of water, was covered 

 with chemical, detrital and volcanic sediments. 



2. The metamorphism of these sediments produced later the schists and gneisses 

 and the fusion of them by the combined action of heat and moisture gave rise to 

 acid igneous rocks. 



3. At the bottom of this series of sediments is a great mass of clastic green- 

 stone which varies to acid rock, by chemical precipitation and by detrital accretion. 

 The metamorphism and fusion of this, and other, intermediate rocks, gave rise 

 to the intermediate schists and igneous rocks. 



4. The basic igneous rocks, gabbro, diabase and their allies, are, in like 

 manner, the result of the same forces acting on the basic rocks (greenstone) whether 

 of the original crust or of the clastic series. 



5. From the most basic to the most acid of the igneous rocks, therefore, there 

 is an unbroken series of minute gradations, both chemical and petrographic, corre- 

 sponding primarily to similar gradations in the clastic rocks, by reason of which it 

 is impossible to establish definite classes or groups separated by constant characters. 



6. Hence all igneous rocks of the Archean, after the first greenstone, are in 

 one heterogeneous family, interlocked in different directions, and incapable of sepa- 

 ration. Every rock sample has its own characters or combination of characters, but 

 these are shared variously with its neighbors, or with its congeners from more distant 

 localities, in almost an infinite series of differences and shades of variation, by reason 



