DIFFERENTIATION OF KEWEENAW AN DIABASES 



435 



feldspars show a wonderful development of the zonal structure 

 (Fig. 3). In the rapid growth of the crystals the zones of calcium- 

 and sodium-rich material have developed mostly at their ends, 

 and they have thus been drawn out to excessive linear proportions. 

 The indices of refraction indicate that the most calcic feldspars 

 form the central zone and the more sodic follow outward. The 

 usual order of rate of weathering of the sodic and calcic feld- 



FiG. 3. — Photomicrograph of a section from a pegmatite dike at Flat Rock Portage 

 showing unusual development of zonal structure in sodium-calcium feldspar (crossed 

 nicols; X40). 



spars does not hold in many of these crystals, as the second zone 

 even with lower index of refraction often shows much more exten- 

 sive alteration than the central zone. The alteration products 

 are epidote and a mineral or mixture of minerals which has yellow- 

 ish polarization colors and is believed to be epidote, kaolin, and 

 zeolites. A peculiar influence of the pegmatitic intergrowth of 

 the minerals is the crystallization ol epidote in some of the zones 



