156 ARCH^AN TIME. 



cording to Logan, between northeast and north-northeast, and mostly the latter; and 

 the strike of the gneiss and schists has the same general course. 



The usual strike of the Archaean rocks of Scandinavia is also to the northeastward, 

 — a fact to be expected where this is the general trend of the mountain range. 



The beds were laid down as sediments over immense continental 

 areas ; and then followed an epoch of uplift, when the horizontal 

 layers were pressed into folds and displaced, on the grand scale ex- 

 plained. Many such periods of uplift may have previously occurred. 

 But it is evident that uplifting and disturbance were not the prevail- 

 ing condition of Laurentian times, any more than they were of later 

 ages. This is proved by the conformability of the various beds to 

 one another in this system of foldings. An age of comparative quiet, 

 allowing of vast accumulations of horizontal strata, even to a thick- 

 ness of 30,000 feet, must have preceded the epoch of disturbance. 



In these primeval times, the ocean worked almost alone at rock- 

 making, without the aid of great rivers to wear off and bring material 

 for its use, as in the later ages ; and consequently rock-making went 

 forward then with extreme slowness. It is obvious, therefore, that 

 the period of comparative quiet, in which the 30,000 feet of rock were 

 deposited, was long. It had the aid of an excessive proportion of 

 carbonic acid in the atmosphere to be carried down with the rains, so 

 that this most efficient of all agents in rock-destruction (p. 705) must 

 have worked with an energy unknown in later time. (Hunt.) 



Alterations : Solidification and Crystallization. — Besides the dis- 

 placements, there was an almost universal crystallization of the old 

 sedimentary beds and limestones ; and now, in place of the sands and 

 clays and earthy limestone layers, the rocks, through this metamor- 

 phism, are granite, gneiss, syenyte, granular limestone, etc. The 

 once massive and earthy limestones now contain in many places 

 crystals of mica, scapolite, apatite, spinel, etc. ; and the limestone itself 

 is in part a white or variegated architectural marble. The argil- 

 laceous iron-ore has become the bright hematite or magnetite ; and it 

 is banded by, or alternates with, schist and quartz, etc., which were 

 once accompanying clay and sand layers. The franklinite (zinc-iron 

 ore) and its associated ores of zinc, often in regular crystallizations, 

 were made from the stratified beds containing impure zinc and iron 

 ores, and were in part limestone strata, like those which afford such 

 earthy ores in Belgium and Carinthia, and near Bethlehem in Penn- 

 sylvania. 



Some of the Archaean rocks have experienced a second or third alteration during 

 the later ages. The potstone or rensselaerite, gieseckite, and part of the serpentine, 

 with some of the associated minerals, are among these later products. The renssel- 

 aerite has been observed under the crystalline form of pyroxene, showing that in part, 

 at least, it has been made out of pyroxene ; and the gieseckite exists under the crys- 



