670 NORMAN L. BOWEN 



The extent to which the comparatively simple relations, exhibited 

 at the contacts described, would be obscured by reintrusion of the 

 mixed magma to a higher level may well be imagined. 



DISCUSSION 



As opposed to the explanation of the origin of the granophyre here 

 advanced, the explanation which would probably first suggest itself 

 is that diabase and granophyre are normal differentiates from a 

 common magma, influenced by gravity. This assumption would 

 neglect the evidence of the Foot Lake contact. Here highly grano- 

 phyric diabase has developed between the slate fragments of the 

 contact breccia. 



Some of the dikes which feed the sills have a width greater than 20C5 

 feet. Assuming a normal differentiation in the sills, it seems likely 

 that it would take place in dikes so wide, giving enrichment in grano- 

 phyric material toward the central portions. Evidence of this could 

 nowhere be found. 



The Foot Lake sill has a thickness of only 50 feet, yet it has the 

 acid rock developed at its upper contact. 



The theory here advanced also accounts logically for the passage 

 of granophyre into adinole of approximately its own composition. 



It has been stated that the diabase in all parts of the sills shows 

 micropegmatite interstices. Possibly much of this material may have 

 an origin entirely different from that of the red interstices close to the 

 granophyre. It should, however, be remembered that the dikes are 

 sometimes without micropegmatite. 



The separation, which is here postulated, of soda-bearing car- 

 bonate waters from the magma is of course a sort of differentiation, 

 but is not in itself sufficient to produce granophyre but only to con- 

 tribute to its formation by inducing alteration of the slaty sediments.^ 

 A necessary conclusion seems to be that granophyre would not have 

 been formed had the country rock of these intrusions been pure lime- 

 stone or pure quartzite just as adinole is not produced in such cases.* 



This conclusion suggests a test of the hypothesis here advanced in 



1 The importance of contact metamorphism in the genesis of igneous rock types 

 has long been advocated by the French school of petrologists. — A. Michel-Levy, Bull. 

 Soc. Geol. Fr., 24 (1896), 123 ff.; also 25 (1897), 367. 



2 W. Hutchings, Geol. Mag., II (1895), 122-63. 



