ARCH^AN TIME. 453 



other minerals have been converted into serpentine ; pyroxene into rensselaerite, a variety 

 of talc ; nephelite into gieseckite ; spinel to hydrotalcite. Another change is that of mag- 

 netite to hematite ; for the great beds of hematite sometimes contain octahedral crystals 

 now consisting of hematite, which, when formed, were octahedrons of magnetite. 



In the ore-beds of the Huronian the layers of ore, jasper, or other materials are often 

 much broken and displaced. The grains of apatite are sometimes more abundant along 

 one tide of an ore- bed than the other, or have some reference to the depressions in which 

 the ore lies (Browne, 1889). The dioryte underlying the ore-bed has been altered in many 

 places to a soft clayey material, feeling soapy, resembling the fluccan of a vein. The 

 underlying rock is sometimes that of a dike, but whether consisting of dioryte or diabase,, 

 it is, in general, probably, as Hunt held, a rock of sedimentary origin. As dioryte and. 

 diabase were very abundant rocks, sediments made from them would have then been com- 

 mon. The broken and otherwise displaced condition of the ore-beds, and the rearrange-- 

 ments of the ore in any depressions that were made, would have been a consequence,., 

 under the results of wider disturbance, of the important fact that in the change of the- 

 carbonate to hematite or magnetite, there is a reduction in the former of one third in hulk, . 

 and in that of limonite to the same ores, a reduction of one half or more, so that larger 

 spaces would have been opened, favoring large displacements. 



The subsequent changes, alluded to above, probably occurred at some later epoch of 

 regional disturbance, in the course of which movement was produced along the plane of 

 the ore-bed. Under the action of the heat from friction siliceous and other solutions, 

 ■would have been formed anew and mineral changes have taken place. 



LIFE OF ARCH.ffiAN TIME. 



A.lthough fossils, according to present knowledge, are absent from Archaean 

 rocks, or are of questionable character, the existence during the later part of 

 the Archaean of aquatic life in its simplest forms is rendered almost certain 

 by the fact that the temperature of the waters was favorable to it, and by 

 the occurrence among the stratified rocks of beds of limestone ; by the 

 abundance in many limestones, and other rocks, of graphite, which constitutes 

 20 per cent of some layers in Canada ; and the presence in the Huronian of 

 carbonaceous shales or slates containing 40 per cent of carbonaceous mate- 

 rials. The life belonged to that division of Archaean time which is desig- 

 nated, on page 441, the Archaeozoic aeon ; and the Huronian rocks represent 

 the latter part of this aeon, if not the whole of it. 



Plants. — Graphite — crystallized carbon — has been made in many 

 later rocks by the alteration of coal-beds ; as at Worcester, in Massachusetts, 

 in Ehode Island, at St. John in New Brunswick, where ferns among the coal- 

 plants have been found in the state of graphite, in Ayrshire, Scotland, and 

 in Bavaria. Even anthracite has been observed in the Archaean rocks of 

 Arendal, Norway. Dawson has remarked that it is scarcely an exaggeration 

 to maintain that the quantity of carbon, in the form of graphite, in the 

 Archaean rocks of Canada is equal to that in similar areas of the Carbonifer- 

 ous system. It is reasonable to conclude, therefore, that although graphite 

 may also be produced by heat, that of the Archaean was largely of organic 

 origin, like that of later rocks. The metamorphism of shales containing 

 carbonaceous materials derived from vegetable, if not also animal, tissues, 



