196 JR. A. Daly — Secondary Origin of Certain Granites. 



rock is practically confined to the upper contact-zone was 

 explained by the collection of the products of digestion at the 

 upper contact by gravitative adjustment in the magma. The 

 low density of the locally formed new magma of assimilation 

 would tend to effect its upward diffusion and the consequent 

 cleansing of the heavier gabbro magma from such acid material. 

 The comparatively slight acidification at the lower contact was 

 attributed to the solution of the quartzite in the period imme- 

 diately preceding the final consolidation of the sill ; at that 

 time the viscosity of the magma was too great to allow of the 

 upward diffusion. 



A principal test for such a hypothesis is obvious. If it be 

 true, there should be other examples among the great basic 

 sills cutting siliceous sediments. It has already been noted 

 that there is actually such acidification of the other gabbro sills 

 encountered between Port Hill and Gateway, and that in them 

 the acidification is always most marked at the upper contact. 

 Much more striking examples have been described with unusual 

 thoroughness in Minnesota and Ontario. The comparison of 

 these other cases is so important that the best established types 

 will here be sketched and illustrated in some detail. The fur- 

 ther discussion of the Moyie sill will be postponed to later 

 pages, in which a synthetic treatment of all the examples will 

 be undertaken. 



B. Occurrences in Minnesota. 



(a) The very able and specially detailed memoir of Bayley 

 on the rocks of Pigeon Point contains, doubtless, the most 

 elaborate argument in favor of the secondary origin of some 

 granites. A brief summary of his facts and conclusions may 

 well be given in the works of Bayley's own outline forming 

 the introduction to his paper. 



"Pigeon Point is the northeastern extremity of Minnesota. It 

 is one of a series of parallel points extending from Minnesota and 

 Canada eastward into Lake Superior. Its backbone is a great 

 east and west dike-like mass of a gray, coarse-grained rock that 

 has always been called gabbro. This consists of phenocrysts of 

 plagioclase in a diabasic groundmass of the same mineral, olivine 

 and diallage, and consequently, it is a diabase porphyrite 



"The rocks through which the gabbro cuts are evenly bedded 

 slates and indurated sandstones of Animikie age. They dip 

 south-southeast at 15 to 20 degrees, except at a very few places 

 near the contact with other rocks, where they are more or less 

 contorted 



" The most interesting features in the geology of the point 

 relate to the series of rocks usually occurring between the gabbro 

 and the clastic beds. Beginning on the gabbro side the series 



