974 THE GEOLOGY OP MINNESOTA. 



["Porphyrel." Transitions from granite 



to porphyry. Granite formed 



from arkose . 



become dense and hard, with much jointing, but here, near the boiler house [Susie island], is a dike of diabase 

 running east and west through the slates. This dike is only ten or twelve feet wide, and, alone, its effect would 

 not be so pronounced, as numerous such dikes have been seen without such effect, but, when its force was 

 united with the deeper-seated action which developed the red granite adjacent, it strongly affected the black 

 slates. In some places the adjacent slates are not only hardened, but on weathering they turn grayish-red, or 

 pinkish, indicating the generation of a red feldspar. In one angular projection, on which the heat of the dike 

 may be supposed to have acted more effectively, there are seen developed some red orthoclase crystals which 

 are scattered through the dense matrix like those of a porphyry. This sample (No. 1854) shows it. This is a 

 step toward showing that the black slates, although less amenable to the change, can be made to take the crys- 

 talline condition by contact with the heated traps, and that they also could be made to assume the condition of 

 porphyry and even red granite." 



In the formation of a granite from a quartzyte it appears that coarse feldspars 

 (orthoclase) are first formed, embracing the adjacent quartz grains poikilitically. 

 It is obvious that under such conditions only the most acid feldspar could form. 

 Rejecting the surplus of silica, such silica is, perforce, remodeled, and, being an 

 easily mobile element under pressure and in presence of heat and moisture, it takes 

 the spaces left between the feldspars, and thus always appears latest to "solidify." 

 This is in part illustrated by No. 784. The feldspars, however, were a little later to take 

 crystalline outlines than some of the quartz, which sometimes appears in bipyramidal 

 shape quite perfect before the clastic structure is lost (vol. iv, pp. 516,517). Compare, 

 also, Nos. 609, 611. 



3. The "porphyrel" of the Upper Keeiratin has been one of the most interesting 

 and most studied of the initial rocks that pass into granite. It is specially described, 

 in connection with this change, under the following: Nos. 1061, 1062, 1769, 2189, 

 2266, 2268, 551G; and in connection with all the rocks named esterellyte, for it is 

 evident that the features of the transition may be studied at either extreme. The 

 megascopic condition of the feldspars in some of the granitic aspects of this transi- 

 tion is illustrated by figure 8 of plate V, taken from No. 776G. 



4. Some of the microscopic phases of this transition from distinctly conglomer- 

 atic rock are mentioned under the following: Nos. 2189, 2245, 2266; and from a 

 siliceous green schist (tuff) under Nos. 1046, 1051, 1052, 1092, 1104. 



5. The transitions from granite to porphyry are geographical and local, due to 

 absence of " phenocrysts" rather than to any difference in the contents or crystalline 

 condition of the rocks. Compare Nos. 1061, 1094, 776G-777aG. 



6. Granite formed from arkose is represented by Nos. 344, 923, 994, 995, 2265, 

 but generally has not been specially noted. Of the rocks along the Kawishiwi river 

 Nos. 994 and 995 specially illustrate the consolidation of granitic debris under 

 aqueo-igneous forces and pressure. It is accompanied by the transformation of all 

 the quartz, the generation of epidote and hornblende, and the general granitization 

 of all the rock. The feldspars, while probably saturated by new feldspathic elements, 

 and considerably clouded, are so identically of the same shapes that they can easily 

 be interpreted as old clastic grains. 



