ARCH^AN TIME. 



451 



flexures. The facts prove that the beds were laid down horizontally over 

 large continental areas, and that denudation in Archaean time, making 

 sediment, was carried on by the ocean along its margins or over partly 

 emerged rocks, and by streams over the land, as it is now. The streams 

 were short in that time of contracted lands, yet well supplied with water 



501. 



under the hot climate. The thickness of the rocks indicates that the 

 amount of deposition and rock-making was enormous. The waters of the 

 small streams and of the ocean owed much of their efficiency to the carbonic 

 acid they contained, this gas being everywhere in excess. Moreover, under 

 these conditions, the formation of beds of iron ore along the shallow margins 

 of the sea and in the shallow waters of the land would have been necessarily 

 one of the great features of the later part of Archsean time ; for the decom- 

 posing iron-bearing rocks would have readily yielded their iron to the attack- 

 ing carbonic acid. Moreover, organic deposits of silica may have accompanied 

 the ore-beds in the basin. 



A thickness of 30,000, 50,000, and 80,000 feet has been attributed to the formations 

 piled up in one series or region. If this means 50,000 feet or more in a single geosyn- 

 clinal area before an upturning, the estimate is to be doubted, for the difficulties of correct 

 measurement of flexed rocks are great. In most cases the facts as to the faults and 

 flexures present cannot be ascertained. A thickness of 50,000 feet of uncrystalline 

 sediments in a geosyncline, during even the later part of Archaean time, militates against 

 all calculations as to the Archgean rate of increase downward in the earth's temperature ; 

 for if the rate were 1° F. for 10 feet of depth, as Thomson has calculated, the bottom of 

 such a geosyncline would have had a temperature of 5000° F. ; or if 1° F. for 25 feet, it 

 would still have had a temperature sufficient nearly for the fusion of basalt. 



ARCH^AN MOUNTAIN-MAKING. 



The stratified rocks of the Archaean are almost everywhere upturned, 

 and generally at high angles, the dip usually being between 30° and 90°. 

 Only portions of the Huronian are nearly horizontal. Moreover, as repre- 

 sented in Fig. 501, they are commonly in flexures, from a few yards to miles 

 in span. Such flexures, whenever they occur, are evidence that great upturn- 

 ings had taken place of the Appalachian kind. The crystallization of the 

 rocks, or their metamorphisrn, was an accompanying result. The rocks of 

 the earliest Paleozoic often lie over them nearly or quite horizontally, as 

 illustrated in the accompanying figure (Fig. 502) from Logan, representing 

 a section from the northern or Canadian side of the Adirondacks. Upon 

 the flexed Archaean rocks lie (2) the Potsdam sandstone of the Cambrian, 



