STRUCTURAL GEOLOGY. 7 



Origin of the fragmental rocks.] 



In the first place, it is apparent that the supply of the basic debris, whether it 

 was derived from erosion or from volcanic vents, was abundant and long continued; 

 and if long continued it indicates that ordinary detrital action was limited or was 

 excluded by the abundance of the supply. If ordinary detrital action had operated on 

 these materials for a long period of time they would have received an indelible stamp 

 of sedimentation and of assortment, such as is seen in the strata of later Keewatin 

 time. It is allowable, therefore, to infer that -the supply was not due so much to 

 erosion of pre-existing rocks as to volcanic activity. 



Since the larger part of these fragmentals consists of basic materials, it is 

 apparent that the early, if not the earliest, magma was a ferro-magnesian one. Such 

 materials, whether in a massive or in a fragmental condition, are, most of all rock 

 substances, liable to alteration, and it is plain that in this case the fragmental and 

 the massive have approximated each other in all outward characters. 



Again, it is a legitimate inference, from the absence or rarity of plain sedi- 

 mentary structures in much of these early tuffs, that they fell, not in the ocean, but 

 on land surfaces. Such land surfaces, exempt from the action of organic matter, yet 

 were subject to powerful atmospheric disintegration, by which the massive and the 

 fragmental rocks, consisting of the same chemical elements and approximately the 

 same minerals, would tend to a uniformity of texture and of structure. 



These rocks, whether massive or fragmental, being the oldest known in the 

 state, have been subjected to all the vicissitudes of subsequent geologic time. They 

 have been pressed, depressed and upheaved and sheared. They have been heated 

 by the ascending isogeotherms and permeated by subterranean mineralizers, and 

 subsequently have been profoundly eroded. No actual volcanic vents have been 

 certainly discovered, but the very evident tuffaceous character of the fragmentals 

 has been met with in several places. These dynamic changes have also tended to 

 unify these two classes of basic rock. 



The plainly detrital rocks of the Archean lie higher, but the sedimentary stamp 

 is also on some of the greenwackes which are described above. The true detrital 

 rocks are graywackes, argillytes, quartzytes and conglomerates, usually quite siliceous. 

 Some of these are in the Lower Keewatin and some are in the Upper. Those in the 

 Lower Keewatin are, as a whole, finer grained than those of the Upper, and embrace 

 argillitic slates, siliceous schists, quartzytes, arkoses and greenwackes, the last forming 

 a link of transition to the older, underlying igneous rocks. Those in the Upper 

 Keewatin are often remarkably conglomeratic and are of great thickness and extent. 

 These occur in association with some argillytes, or black slates, and they also grad- 

 uate into quartzytes and to sericitic schists. 



All of the true detrital rocks of the Archean show the presence of oceanic 

 waters, and the structure which oceanic distribution always implants on detrital 



