GENETIC RELATIONS OF SOME GRANITIC DIKES 645 



2. Application to Pegimatites 



As applied to the giant granites, tlie pegmatites, tlie ultra ooarse-graiued 

 rocks, the principle oieutioned suggests that the rock broth, or magma, from 

 which they formed was not so different in its physical conditions, such as 

 temperature, pressure, and gas saturation, from the surrounding rock. They 

 might accordingly be formed by a raising of the rock to a point where any part 

 of its constituents became fluid, or. what amounts to the same thing, where 

 its inherent rock moisture has its solvent power markedly increased. This 

 might be the origin of what might be called segregation pegmatites. But, on 

 the other hand, we may consider them products of an ordinary granitic magma, 

 occurring in the granite mass or in its contact zone. 



The suggestion would then be that they should be a relatively late product 

 of it, after its contact zone had Ijeen heated up and after the magma had 

 lost some of its original fluidity, heat or gas, and was on the point of crystal- 

 lizing — indeed, had perhaps largely crystallized already. leaving only a residual 

 anchieutectic* magma. 



3. Observations in the Huron Mountains 



The foregoing reflections came to me from some observations in the Huron 

 mountains. The locations where these facts may be obser^-ed are so very 

 numerous that they can hardly be particularized, and in fact Professor A. E. 

 Seaman, of the Houghton College of Mines, who has vastly wider experience 

 than I of this region, says, also, that the pegmatites cut the aplites, so far as he 

 has seeu.t 



The country rock is a series of banded gneisses and hornblende-schists and 

 amphibolites, Kee\Vatin-Laurentian, forming anticlinal regions between the 

 basins of Huronian rocks. The country was mapped by the first surveyors 

 as "syenite and gneissoid granite." and has since been mapped with the Azoic, 

 Laurentian, and Archean. At times the banding is well marked, strikingly 

 like that of a series of sediments.t 



The amount of hornblende varies greatly. The strike of the banding is 

 pretty constantly west northwest— that is, parallel to the axis of the synclinal 

 of Huronian rocks that lies south. 



This country rock is cut by various classes of dikes. 



(1) In the first place, it is cut by the group of granitic dikes which we wish 

 to study. 



(2) They are cut by a group of uralite diabases, epidiorites, etcetera, which 

 are probably of Huronian age. 



(3) Latest are diabases, like that of Marquette, which are presumably 

 Keweenawan.§ 



• Vogt's word — almost eutectic. Compare also my paper on the role of possible eutec- 

 tics in rock magmas. Journal of Geology, 1904, pp. 86, 89. 



t However, this paper is based mainly on notes and sketches on Huron mountain, 

 section 39, township 52, range 29, and sections 24 and 25 adjacent ; also mount Homer, 

 section 31, township 52, range 28. near Mountain lake ; also Pine lake, in section 28^ 

 township 52, range 28, especially, and section 4, township 52 north, range 27. 



t So around Ives lake. 



§ See Geological Survey of Michigan, vol. vi. part i, chapter x. 



