46 THE POPULAR SCIENCE MONTHLY. 



It will be observed, in regard to these theories, that none of them 

 suppose that the old gneiss is an ordinary sediment, but that all re- 

 gard it as formed in exceptional circumstances, these circumstances 

 being the absence of land and of sub-aerial decay of rock, and the 

 presence wholly or principally of the material of the upper surface 

 of the recently hardened crust. This being granted, the question 

 arises, Ought we not to combine these several theories and to believe 

 that the cooling crust has hardened in successive layers from without 

 inward ; that at the same time fissures were locally discharging ig- 

 neous matter to the surface ; that matter held in suspension in the 

 ocean, and matter held in solution by heated waters rising from be- 

 neath the outer crust, were mingling their materials in the deposits of 

 the primitive ocean ? It would seem that the combination of all 

 these agencies may safely be invoked as causes of the pre-Atlantic de- 

 posits. This is the eclectic position which I endeavored to maintain 

 in my address before the Minneapolis meeting of the American Asso- 

 ciation in 1883, and which I still hold to be in every way probable. 

 That these old gneisses were deposited not only in what is now the 

 bed of the Atlantic, but also on the great continental areas of America 

 and Europe, any one who considers the wide extent of these rocks 

 represented on the map recently published by Professor Hull can 

 readily understand. 



It is true that Hull supposes that the basin of the Atlantic itself 

 may have been land at this time, but there is no evidence of this, more 

 especially as the material of the gneiss could not have been detritus 

 derived from sub-aerial decay of rock. Let us suppose, then, the floor 

 of Old Ocean covered with a flat pavement of gneiss, or of that mate- 

 rial which is now gneiss, the next question is, How and when did this 

 original bed become converted into sea and land ? Here we have some 

 things certain, others most debatable. That the cooling mass, espe- 

 cially if it were sending out volumes of softened rocky material, either 

 in the exo-plutonic or in the crenitic way, and piling this on the sur- 

 face, must soon become too small for its shell, is apparent ; but when 

 and where would the collapse, crushing, and wrinkling inevitable from 

 this cause begin ? Where they did begin is indicated by the lines of 

 mountain-chains which traverse the Laurentian districts ; but the rea- 

 son why is less apparent. The more or less unequal cooling, hardening, 

 ami conductive power of the outer crust we may readily assume. The 

 driftage unequally of water-borne detritus to the southwest by the bot- 

 tom-currents of the sea is another cause, and, as we shall soon see, most 

 effective. Still another is the greater cooling and hardening of the 

 crust in the polar regions, and the tendency to collapse of the equa- 

 torial protuberance from the slackening of the earth's rotation. Be- 

 sides these, the internal tides of the earth's substance at the times of 

 solstice would exert an oblique pulling force on the crust, which might 

 tend to crack it along diagonal lines. 



