188 R. A. Daly — Secondary Origin of Certain Granites. 



minerals. The structure of the rock is typically hypidio- 

 morphic-granular. 



Already in those sills that range from 400 to 500 feet in 

 thickness, the gabbro is acidified near its upper contact. The 

 change from the normal composition is seen in the great increase 

 of biotite, orthoclase, microperthite and interstitial quartz. 



49°ti> Lat 



Fig. 1. Map of Moyie Sill, taken from plane-table sheet of the Interna- 

 tional Boundary Commission. 1. Moyie argillite. 2. Kitchener quartzite. 

 3. Hornblende gabbro sill. 4. Acidified (granite) zone of sill. 5. Creston 

 quartzite. 6. Alluvium. Conventional sign for strike and dip. Scale : 

 one inch = about one mile. 



Biotite and quartz then assume the proportions of essential 

 minerals. The quartz is characteristically in poikilitic relation 

 to all the other constituents except orthoclase and microper- 

 thite, with which it is in true micrographic intergrowth. From 

 this micropegmatite-bearing phase of the intrusive there is a 

 gradual transition to the normal gabbro which thus comjDoses 

 the lower three-fourths or four-fifths of the sill. 



