368 Prof. J. D. Dana on some Results of 



These iron-bearing kinds of Archaean metamorphic rocks 

 much exceed in amount the granitic kinds that are free from 

 iron. Moreover in these Archaean formations, the granitic as 

 well as the iron-bearing, there are immense beds of iron-ore, as 

 seen in New York, Canada, Northern Michigan, Missouri, 

 Sweden, and Norway, the thickness of single beds frequently 

 exceeding a hundred feet ; and thus, although the surface over 

 which Archaean rocks are exposed is relatively small, its iron is 

 in vast amount. In fact, unlike human history, the earth's iron 

 age was its earliest. Now, since these great iron-ore beds are of 

 sedimentary or marsh accumulation (for they occur interstrati- 

 iied with quartzite, chlorite, schist, syenitic schist, and other 

 metamorphic rocks), the iron was gathered from the preexisting 

 crust-rocks ; they therefore prove that iron was a very common 

 ingredient in the original fused material of the surface of the 

 liquid globe. 



(/) We hence have reason for the inference that the original 

 fused material contained largely the ingredients of the iron- 

 bearing rocks dolerite, peridotite, diorite, hyposyenite, besides 

 trachyte and the related kinds, and, perhaps, in small proportions 

 those of the quartziferous trachytes, if not of granite and 

 syenite. 



It is not certain from present knowledge whether the slow 

 cooling must not have made hornblende throughout the crust- 

 mass in place of the akin species augite. Yet the considera- 

 tions mentioned on the preceding page suggest that augite may 

 have characterized the basic portions of the crust, and horn- 

 blende the smaller acidic portions; and if so, the prevailing 

 rock is strictly doleritic. 



(g) In view of the large proportion of iron, the mean specific 

 gravity of the true crust can hardly be less than 2*9, and probably 

 it is as high as 30. 



(h) The method of distribution of the basic (doleritic*) and the 

 less abundant acidic (or trachytic) kinds in the earth's outer 

 viscid layer before solidification — whether in separate layers, the 

 latter over the former, as Durocher urges, or whether in sepa- 

 rate local streams or regions made by the boiling movements 

 and the great oceanic-like currents in the liquid mass, through 

 the principle of liquation on a large scale — need not be here 

 considered. I merely add that observed facts seem to be best 

 explained on the latter view, since the existence of the two 

 layers is not proved by the study of the Archaean terrains. If 

 it existed at the beginning of solidification and the trachytic 

 layer was thick enough not to have been obliterated by cooling 

 before the era of the more recent trachytic ejections over the 

 great Pacific slope, the constituents of the iron-bearing or dole- 



