: " 



X 



PART I. 



STRUCTURAL GEOLOGY. 



BY N. H. WINCHELL. 



The following sketch is based on the field facts, which are given in volume iv, 

 and on the petrographic descriptions that follow in this volume. 



THE ARCHEAN. 



Di'Jhiition of the term. As here employed the term Archean embraces that great 

 series of crystalline and metamorphic rocks which lies below the Taconic or Lower 

 Cambrian, and which is separated from the overlying rocks by a violent non-con- 

 formity. This horizon of separation is not known, nor presumed, to be immediately 

 below the Olenellus horizon, as lately defined by the United States Geological survey; 

 but it is probably considerably older than the Olenellus (or the Paradoxides*) horizon, 

 although in strata probably conformable with strata of that horizon. These crys- 

 talline rocks, in whole or in part, have borne the names of Primary, Laurentian, 

 Pre-Cambrian, Azoic, Eozoic and Fundamental Complex, but the term Archean, sug- 

 gested by Dana, seems appropriate, and also is the most used.f As defined by him, 

 it included Huronian and Laurentian. Owing to uncertainty as to the significance 

 of the Canadian terms (Huronian and Laurentian) they are not employed in this 

 discussion except for purposes of reference, although it now appears that the original 

 Huronian embraced a part of the Taconic and a part of the Archean, while the 

 Canadian Laurentian seems to consist largely of igneous rocks and of metamorphic 

 elastics of different parts of the Archean. For the details the reader is referred to 

 the plates and the special chapters accompanying them included in volume iv. 



The fragmental rocks of the Archean. 



Nature of these frag mentals. A large part of the fragmental debris that enters 

 into the composition of the Archean, at least in its lower portions, is of volcanic 

 character. It consists of fragments of minerals and of rocks that, even after more 



* According to Prof. G. F. Matthew the Olenellus and the Paradoxides horizons are not always distinct, but probably 

 blend in their stratigraphio and faxinal characters. American Oeologixl, xix, 396, 1897. 



iThc recent restriction of the term Archean, by Van Hise and Bayley, in their monograph on the Marquette iron-bearing 

 nicks, to the rocks supposed to be at the bottom of the "basement series" of Irvins,'. i. <'., those that are without evidence of water 

 drpi isition, introduces an element of confusion of which the reader should take note. It is a wide departure from Dana's definition 

 of the term Archean. 



