986 THE GEOLOGY OP MINNESOTA. 



[The intermediate rocks. 



graphic and structural, when apprehended after long field examination and exhaust- 

 ive microscopical study, is toward a unity of origin, as to manner and date, of all 

 these variations. 



The intermediate rocks. The question may arise what was the source of the 

 intermediate crystalline rocks? Granite is at one end of the scale and gabbro and 

 diabase at the other. It is true that these extremes have been most studied, both in 

 the field and in the laboratory, and that the foregoing discussion applies essentially 

 only to them. That was necessary. The prime types must first be explained on 

 some rational hypothesis, and the intermediate rocks and all other attendant facts 

 must be assumed to group themselves in some way in rational dependence on them. 

 That seems to be the case. The intermediate rock types, both in the Archean and 

 in the Taconic, blend into the end types. There is no abrupt distinction except 

 locally, and everywhere vanishing distinctions. 



If we may judge of the Archean rock terranes before metamorphism by what 

 we see of the Archean now unmetamorphosed, the original rocks consisted primarily 

 of very acid and very basic. There were graywackes (and slates) and greenstones. 

 But there were also basic variations in the graywackes as well as siliceous gradations 

 in the greenstones. Proportionately these geographic belts of transition were of 

 minor importance. The change from a basic clastic rock to an acid one must have 

 taken place somewhat as it does now in the Archean sedimentaries, rather rapidly, 

 and the clastic rocks of intermediate character must have been as rare as they 

 appear to be to-day. Hence, it is reasonable to suppose that, on metamorphism such 

 as presumed above, the resultant igneous rocks would be prevailingly either acid or 

 basic, but that in places there would appear smaller amounts of intermediate igneous 

 rocks. 



Consequences of this hypothesis, if true. One of the first and most obvious results 

 springing from this discovery of the origin of Archean igneous rocks, whether acid 

 or basic, is the non-applicability of the theory of differentiation of magmas. What- 

 ever may be the case in other parts of the world, where igneous rocks have been 

 examined from that point of view, there is certainly no standing room for its use in 

 the Archean and Taconic of Minnesota. 



Another result of this hypothesis, if true, is to lessen the value of chemical 

 analysis of igneous rocks. Great variations can be found within the muscovadyte 

 belt, running from peridotyte and pyroxenyte to quartzyte and to magnetyte. All 

 these variations can be referred to the varied composition of the original Keewatin. 



It also has a bearing on the origin of all the later igneous rocks. The universal 

 greenstone crust would at any epoch in geological history furnish a diabase or a 

 gabbro, should it be affected by such dynamic or other agents as to produce fluidity, 



