B. A. Daly — Secondary Origin of Certain Granites. 201 



as a sill," but adds the remark that " while the contacts of the 

 quartzites with the red rock and gabbro so far as they were 

 seen are vertical, it does not necessarily follow that they are 

 vertical with depth." He continues : " I have no means of 

 knowing the date of the intrusion. With respect to the tilt- 

 ing (of the quartzites) my guess is that the intrusion was prior 

 to the latest tilting, but later than an earlier tilting lake ward." 



The possibility that the Pigeon Point eruptive is either a 

 true sill only locally breaking across the bedding of the sedi- 

 ments or at any rate dips as a whole to the south-southeast- 

 ward, is further suggested by the analogy of the many 

 undoubted sills of gabbro cutting the southerly to southeasterly 

 dipping Animikie of Minnesota. Some of these sills have 

 likewise zones of soda granite lying between the gabbro and 

 the sediments on the southerly flank of the gabbro. Thus in 

 those cases the sediments dip under the gabbro on the one 

 side of the eruptive body and away from the granite on the 

 other side. 



Bayley's full and trenchant argument for the contact origin 

 of the soda granite and granophyre need not be repeated. 

 The independent origin of the acid rock is rendered highly 

 improbable by the occurrences of the intermediate rock lying 

 directly between the gabbro and the sediments without the 

 intervention of the true granite or granophyre. 



The efficiency of contact-shattering in aiding the digestion 

 of the slates and quartzites is strikingly manifest in Bayley's 

 descriptions. 



" Very close to the red rock appears a belt in which the various 

 rocks are in the most complicated relations imaginable. In the 

 eastern portion of the point this belt is well seen on the southern 

 shore, about one-third of a mile from the end of the point. (See 

 PI. XYI.) Here the red rock is exposed in low cliffs, and in it 

 are small, sharp slate and quartzite inclusions, into which the red 

 rock penetrates in every direction. The exact line of contact 

 between the red rock and the bedded fragmentals cannot be 

 detected, as they appear to merge gradually into one another, the 

 latter becoming redder and redder as they approach the former, 

 which penetrates them in veins and dikes, and finally includes 

 numerous pieces in such a way as to yield a good eruptive 

 breccia." 



" Some of the inclusions are very sharp and but little altered, 

 while others are partially dissolved, and are surrounded by con- 

 centric zones, resulting from the action of the red rock upon the 

 material of the inclusion, and the reciprocal effect of the partially 

 dissolved inclusion upon that portion of the red-rock magma 



immediately contiguous to it Thus it would seem to be a 



fact beyond controversy that the red rock is the immediate cause 

 of the alteration noticed in the fragmental rocks and of the brec- 



Am. Jour. Sci. — Fourth Series, Vol. XX, No. 117. — September, 1905. 

 14 



