38 PROCEEDINGS OP THE NATIONAL MUSEUM. ' vol.66. 



considered that the surrounding ssdiments played an important part in the for- 

 mation of granophyric bodies at the upper surface of the sills. This opinion was 

 arrived at principally because of the difficulty of picturing any process of pure 

 diflferentiation whereby a quartzose rock could be formed from basaltic magma. 

 With this difficulty removed the writer has no hesitation in concluding that the 

 granophyre and the micropegmatite interstices of the diabase were formed after 

 the manner detailed in the present paper and that interchange of material between 

 the granophyre and adinolized sediment was a subsidiary process contributing 

 to the soda-rich nature of the border phases. 



There is no support, in the relation of the later differentiated types 

 of material at Goose Creek, of the theory of immiscible separation 

 of liquids in the magma. On the contrary, the clearly intrusive 

 relationship of the later rocks in many cases shows that the surround- 

 ing diabase must have been almost completely crystallized although 

 it unquestionably was at an elevated temperature and quite prob- 

 ably was more or less pasty rather than rigidly solid. It is clear 

 that the masses of diabase pegmatite, albitic pegmatite, and quartz 

 albite rock are the result of a difierentiative process which took place, 

 locally, in a magma chamber wherein the magma, as a whole, cooled 

 and crystallized too rapidly to permit of general differentiation. 

 Obviously some special factor was active in the control of events and 

 it is pertinent to inquire into the nature of this factor. 



The most strikingly conspicuous feature of these unusual rocks is 

 their extremely coarse texture, and it has been stated above and 

 substantiated by quotations, that the most probable cause of this 

 coarsening was water (or other volatile constituents) of the magma. 

 It is noteworthy that in a great majority of the described examples 

 wherein extremely alkalic differentiates have resulted from diabasic 

 magma, especially in smaller masses where, as here, the main sill 

 mass shows little evidence of differentiation with settling of crystals 

 or conspicuous zoning, the same sill contains a.bundant evidence of 

 the presence of more than ordinary amounts of volatile material, 

 notably water, as indicated by masses and dikes showing extremely 

 coarse textures and by intense hydrothermal alteration of the rock 

 immediately following their consolidation. Thus in the Gowganda 

 Lake region there are examples of extreme coarsening of grain in the 

 diabase and of intense adinolization of the enclosing rock at the con- 

 tact. In the Duluth gabbro the coarse structures are conspicuous 

 while the red rock is characterized by a porous texture and kaoliniza- 

 tion of the feldspar and uralitization of the ferromagnesian minerals. 

 It seems that differentiation resulting in moderate amounts of silicic 

 rocks is greatly faciliated by a richness of the magma in dissolved 

 water. The source of this water is problematic. It may have been 

 original in the magma when it was intended or it may have been 

 subsequently derived in considerable part from the intruded rocks. 



