62 W. S. Bayley — Rocks of Pigeon Point, Minnesota. 



In these results can be traced the gradual transition from 

 the basic gabbro, rich in calcium and magnesium, and poor in 

 potassium, to the acid keratophyre, which is poor in calcium 

 and magnesium and rich in potassium. We can hardly 

 imagine the conditions under which a rock of the composition 

 of the gabbro (IV) could be changed into a rock of the com- 

 position of the intermediate rock (VII), by means of solutions* 

 emanating from the keratophyre, unless these solutions con- 

 tained in them the materials of the keratophyre in about the 

 proportions in which they are present in that rock, a sup- 

 position which is not at all probable. 



It would seem, then, that we are justified in regarding the 

 intermediate rock as due to the fusion and recrystallization of 

 the materials of both the keratophyre and the gabbro, in con- 

 sequence of the irruption of one of these rocks into the other 

 at some considerable depth below the surface of the earth, 

 where the conditions were such as to produce a rock with the 

 characteristics of a plutonic rock. In other words the inter- 

 mediate rock is the result of deep seated contact action. 



Analyses of Irving's augite-syenites are not given, so that a 

 comparison of their composition with that of the intermediate 

 rock (No. 11,209) cannot be made. Their thin sections, how- 

 ever, as has already been stated, exhibit a very close similarity 

 to many of those of the contact rocks. Further, those with 

 these characteristics are always, so far as could be determined, 

 in close association with gabbro, and in many cases are also 

 very near a more acid red rock resembling the quartz-kerato- 

 phyre in one of its phases. 



The augite-syenites of Irving, then, may be divided into 

 two classes, those which are like the quartz-keratophyre, de- 

 scribed above, and those which are similar to the contact rock. 

 In neither case are they altered eruptives, in the sense that 

 they owe their present characteristics to the alteration of a 

 more basic eruptive. They have both resulted from the solidi- 

 fication of a molten magma. 



Of course it is not affirmed that no alteration has taken 

 place in any of the augite syenites, for such is not the case. 

 Some of them have suffered the kaolinization of their feld- 

 spar, and the chloritization of their augite and mica, with the 

 production of secondary silica. Their most characteristic 

 properties, however, are not due to this alteration, but are due 

 to the chemical composition of the magma by whose cooling 

 they were formed. 



* Cf. American Geologist, June, 1 888. p. 343. Messrs. Herrick, Clarke and Dem- 

 ing: Some American Norites and Gabbros. 



