220 SMITH AND MENDENHALL — TERTIARY GRANITE 



evidence — the areal distribution of the granite and the contact phe- 

 nomena. 



Distribution of the Granite 



areal extent 



The granite occurring within the limits of the Snoqualmie quadrangle 



is simply the southeastern border of a large mass, which extends for 

 some considerable distance both to the west and to the north. Its ex- 

 tent in the Snoqualmie quadrangle where it has been mapped is about 

 10 miles east and about the same distance south of the northwestern 

 corner. This mass of plutonic rock undoubtedly covers an area measur- 

 ing not less than 100 square miles in extent. It is therefore an impor- 

 tant element in the architecture of the Cascade mountains. 



DIKES 



Within the area studied there is one apophysis from the granite mass 

 which is more than one mile in length. Another dike in the same part 

 of the area is less regular in character. While only a few hundred yards 

 in width for a mile or more, it swells out into boss-like masses of granite 

 at two points, these masses being about a mile in diameter. The whole 

 body thus roughly resembles in outline a dumb-bell, and is connected 

 by a narrow apophysis to the main mass. A third dike of granitic rock 

 extends up into the overlying andesitic lava, and thus clearly shows the 

 relative age of the volcanic rock and the plutonic. Smaller dikes occur 

 both along the contact and at some distance within the areas of sedi- 

 mentary or of volcanic rock. In several cases the connection of dikes 

 of this latter class with the main granite mass has not been traced, 

 though such connection probably exists. 



The rock of these dikes is a granite porphyry — fine grained for the 

 most part and very rich in quartz. The abundant bipyramidal quartz 

 phenocrysts with the almost felsitic groundmass makes this porphyry a 

 noticeable rock in the field. 



Contact Metamorphism 

 endomorphic phenomena 



The textural variations within the granite mass offer strong evidence 

 of its intrusive nature. At a distance of 2 miles from the contact, as on 

 Denny mountain, the granite is of medium grain and shows the typical 

 granitic texture. As the contact is approached, however, a gradual change 



