BAKED CLAYS AND NATURAL SLAGS 411 



a reddish tint. In passing from the reddish argillite towards the 

 center of the vein, the contact zone shows a gradual decrease in the 

 normal shale constituents and a development, in increasing amount 

 and coarseness, of purplish-blue, pleochroic cordierite. Next the 

 coarser part of the vein this mineral is present to the exclusion of all 

 others. In this contact zone the red iron oxide of the argillite has 

 been wholly reduced to magnetite. The central portion of the vein 

 is a somewhat vesicular, holocrystalline mass, consisting of abundant 

 magnetite in irregular masses, some hematite, usually lining the vesi- 

 cles and following fractures, and abundant cordierite, feldspar, and 

 pyroxene. The cordierite occurs in good-sized grains, some of which 

 have a very irregular outline, while others show very definite crystal- 

 line forms, occurring in short prisms whose very perfect hexagonal 

 cross-sections are the result of characteristic repeated twinning, as 

 is shown by the optical properties. The feldspar occurs in narrow 

 lath-shaped crystals with irregular terminations, and often with 

 irregular lateral boundaries. No crystals were found which showed 

 more than two twinning lamellae. The index of refraction is slightly 

 above that of the balsam, and about equal to that of cordierite. 

 This character, and the low extinction angles (less than 3 ), fix its 

 composition as oligoclase. Pyroxene occurs, sometimes in grains, 

 but mainly in long, prismatic crystals, whose length ranges from ten 

 to almost fifty times their width. The pleochroism is moderately 

 strong; the colors for rays vibrating parallel to the prism length 

 range from deep green to greenish-yellow, with occasional portions 

 of the prism which are reddish-brown ; for rays vibrating perpendicu- 

 lar to the prism, the color is usually greenish-yellow. Cross-sections 

 show typical pyroxene cleavage, and the extinction angles range up 

 to 32 . 



5. A small proportion of the beds are fine-grained, porous sand- 

 stone which, upon baking, assumes the brilliant colors observed in 

 the clayey members. Microscopically the slag is seen to penetrate 

 this porous rock so thoroughly that it is impossible to draw a sharp 

 line between the slag and the original sandstone. The sandstone, 

 consisting of small angular quartz grains, fine argillaceous material, 

 and hematite in scattered grains and as a fine coating on the other, 

 minerals, passes into a glassy mass inclosing scattered quartzes and 



