STRUCTURAL GEOLOGY. 25 



Genesis of the Archean. The greenstones.] 



time much more accurate knowledge has been reached as to the nature and succession 

 of all the parts of the crystalline rocks. The study of the igneous rocks proper since 

 this division of the inquiry has been conducted by Rosenbusch, Teall, Iddings, 

 Brogger, Michel Levy, Cleikie, Becke, La Croix, Lang, Lewinson-Lessing and others. 

 The examination of the order of succession of the Archean rock masses has been 

 carried on chiefly in the United States by the officers of the U. S. Geol. Survey, and 

 of some of the state surveys. 



On theoretical grounds, Dolomieu maintained, in the latter part of the last 

 century, the existence, " beneath the granitic substratum, of a liquid layer which 

 gives origin to basic rocks and lavas. A similar view was developed later by Phillips, 

 Durocher, Bunsen and Streng, " who have imagined a separation of the liquid layer, 

 or the matter at the surface of the cooling globe, into two layers, an upper acidic one 

 corresponding to granites and trachytes, in which, besides alumina and an excess of 

 silica, lime, magnesia and iron oxide are present in very small quantities, and potash 

 and soda abound; and a lower basic one corresponding to doleryte and basalt, in 

 which lime, magnesia and iron oxide abound with an excess of alumina, and but little 

 alkali." These two constitute the trachytic and pyroxenic magmas of Bunsen, who 

 derived his ideas (1851) from a prolonged study of the volcanoes of Iceland. Bunsen 

 also believed that various intermediate rocks are produced by a mingling in different 

 proportions of these two separated magmas. 



Waltershausen (1853) criticised Bunsen's conclusions and maintained that while 

 there is no distinct separation into basic and acidic magmas, there is still a gradual 

 passage downward from acidic to basic, even increasing to a metallic interior with 

 the metals and minerals arranged according to their densities. This was adopted 

 and extended by Macfarlane(1864), and approved by Richthofen and by Zirkel (1866). 

 According to this theory the earliest igneous eruptions were not of basic character, 

 but came from near the exterior, and were feldspathic. Later igneous rocks became 

 progressively more basic, coming from deeper and deeper reservoirs where the more 

 basic elements reside, thus accounting for the basic nature of the late eruptions. 



Von Gotta (1858) proposed the idea that below the siliceous crust, made up of 

 granites, gneisses, etc., there exists a mass of more basic rock material mainly in the 

 state of fusion, and that this sub-crust mass is the source of the basic eruptives. 

 These eruptives become modified by the incorporation of more or less of the acid 

 crust in the process of extrusion. 



Durocher (1857) also shows the existence of two distinct magmas, one acid and 

 one basic, and that intermediate rocks of eruptive origin are due to contact and 

 mingling of these two magmas. Gravity determines the relative positions of these 

 magmas, the basic being below the acidic. 



