62 THE GEOLOGY OP MINNESOTA. 



[Effect on the Aniiniki*- the red rock . 



That this red rock is in some way dependent on the Keweenawan is indicated by its 

 non-occurrence elsewhere than in connection with the Keweenawan. That in all its 

 phases, which sometimes reach a crystalline condition equal to that of granite, it has 

 resulted from the fusion of an acid rock, is a legitimate inference. It may hence 

 be accepted as a fair illustration of the oi'igin of granite. It is the same which 

 Irving called augite-granite, although augite is a more rare mineral in its consti- 

 tution than has been supposed. It is not to be presumed that the fusion of the 

 Animikie was necessarily the only source of this late granite. The tremendous 

 agitation which gave origin to the gabbro probably affected the rock on which the 

 Animikie lies, and it may have been fused in like manner and its contribution to 

 the fused mass, when acid, went to swell the volume of the red rock, and when basic 

 may have been added to the gabbro. 



Neither is it to be assumed that this fusion was a mere surface effect, due to the 

 overlie and heat generated by the gabbro on the Animikie from above downward. 

 Indeed, it is noteworthy that the bosses of the red rock, where they seem to be in 

 situ of their formation, penetrate to greater depths and show by their behavior that 

 their primitive seats are below the surfaces where they occur. Sometimes the red 

 rock is struck, as at the mining shaft on the island south of Pigeon point, below a 

 greater or less thickness of Animikie strata which are but slightly altered. When 

 the red rock can be traced by successive steps of petrographic transition into the 

 Animikie in place, the red rock concerned is not a granite, but a red quartz- 

 porphyry, or keratophyre, as in the low red-rock knob on the south side of Pigeon 

 point. The greater crystallization is not on the side toward the Animikie, but on 

 those sides most remote. Most of the red rock dikes and the surface flows are to be 

 considered, therefore, as peripheral manifestations of a greater volume, and as 

 suddenly cooled portions escaping from a magma which, congealing slowly, gave 

 source to the augite granite. Hence the Animikie was fused from below by the 

 gabbro mass and not from above. The gabbro, as we see it, lies on and penetrates 

 the Animikie, but the action of this intrusion, and the distribution of the red rock, 

 show that the intensity of alteration was seated below all the Animikie of which 

 we have knowledge, and hence that the bottom of the Animikie suffered the trans- 

 formation before the top was involved. Therefore, while in the main contemporary 

 with gabbro, sometimes penetrating it, sometimes penetrated by it and sometimes 

 mixing with it so intimately as to constitute intermediate rocks, such as " orthoclase 

 gabbro" of Irving, and " hornblende gabbro" of Streng,* and although at numerous 

 points the dependent genetic relation which the red rock bears to the fusion of 

 the Animikie can be shown by ocular demonstration, it must be allowed that both 



* Eleventh Annual Report, Minnesota Survey, p. 51. 



