ON CORDIERITE AS CONTACT MINERAL 317 



kind of spotted slate is followed toward the Granite by a narrow zone 

 of altered slate containing the characteristic crystals of Cordierite. 

 The Cordierite- bearing slate is found close to the Granite and usually 

 no particular indication of metamorphism can be detected other than 

 by the presence of these crystals, or else by the compact texture, like 

 that of "Ilornfels.' which it assumes. This inner zone is comparatively 

 narrow in breadth, and exposures of some 10 mein* may be seen 

 cropping out at the immediate vicinity of the Granite, while the 

 outer zone is usually broader, being double or treble of the inner. 



Microscopically examined the ground mass of the Cordierite 

 slair differs, in no essential respect, from the spotted slate already 

 mentioned, consisting of minute quartz grains, ami a black mica, with 

 irregular dots of opaque carbonaceous matter. 



[ am indebted to Mr. T. Xasa i'w the following chemical analyses 

 of the slate, which were made in the laboratory of the Geological 

 Survey. 



I. II. III. 



Si 0, 66-10% 67-50% 74*50 



AL Oj 16-39 



Fe <> 4-65 



Fe, -30 



' 'a 1-60 



Mg 2-41 



IC, 3-3(5 



Na, <> 1-86 



H,0 3-60 



100-^7 100-16 99-92 



I. is the slate with the Cordierite crystals, which showed a slight 

 sign of alteration, assuming a greenish tinge. The Ijarge 



