GENETIC RELATIONS OF SOME GRANITIC DIKES 



647 



same tendency to follow straight lines or cnt irregularly across the country 

 rock, tcith perhaps no exception the coarser dikes cut the aplltes. 



4. Conclusions 



The inference seems warranted that the grantic dikes, aplites and pegmatites 

 together, represent one period of invasion of the Keewatin hornblende schists 

 and gneisses by the granitic magma.* 



In the beginning the difference of conditions of country rock and magma 

 was considerable. Still the absence of fine-grained margins shows that the 

 conditions of solidification of the granitic magma (pressure, temperature, 

 mineralizers) were nearer the conditions of the country rock than those of. 



Figure 1.- 



28 INCHES 



-Structure of a granitic Dike on Mouiit Homer. 



The quartz grows out for 4 Inches (100 millimeters) from the margin in rude 

 forms. The rest of the way is mainly filled with patches of graphic granite, 

 black ; feldspar white. 



prismatic 

 Quartz is 



the magma, or at times just about half way. and then the aplitic dikes formed. 



Later the temperature of the country rock rose, or it otherwise approached 

 the conditions of liquefaction for granite. The granitic magma also cooled. 

 At any rate, the interval decreased! from the conditions of consolidation of 

 the dikes to the conditions of the country rock, and so the later pegmatites 

 are coarser. If the granitic magma did not also cool, thej^ would become 

 even more than the aplite uniform in grain from center to margin except so 

 far as it was modified by a tendency of substances existing in excess of the 

 eutectic to crystallize out earlier, or a tendency to grow first from the walls, 

 as in veins, the same tendency that makes nuclei hasten the crystallization of 

 an undercooled fluid. 



Such tendencies may explain the coarser grain ;it the margin (see figure 1) 

 in part, but it may also be explained by supposing that the granitic magma 

 cooled as fast as the country rock heated, so that the conditions of consolida- 

 tion were about half way between them. 



Criticism has been made of my studies of grain that I have not allowed 

 enough for imdercooling and varying velocitj' of crystallization. t 



I think it will be found that I have always recognized that other factors 

 than rate of cooling were of importance, and my matliematical work only 

 gives a first approximation, but I have insisted that observation shows that 

 often it has been of leading importance, and my results have been obtained 



♦Compare Rosenbusch, "Elemente der Gesteinslehre," p. 220. 



t V of my annual report. Geological Survey of Michigan for 190.'3, pp. 211 and 212. 



t Doelter : Petrogenesis, p. 45. 



