GEOLOGY OF SARATOGA SPRINGS AND VICINITY 12/ 



All the slides contain crystals of black, opaque material. There 

 are occasional small crystals of pyrite. There is much minute, 

 dustlike material, especially in the glass, which may be magnetite, 

 though it wholly lacks metallic luster, perhaps because of its min- 

 uteness. There is also much somewhat coarser material, of irreg- 

 ular outline which we should like to identify as graphite. The 

 chemical analysis indicates a large content of carbon in some form, 

 but in reflected light this material does not show a metallic luster 

 such as graphite possesses, but takes on an even, whitish sheen 

 which we are unable to compare with anything we have ever seen 

 in thin section. It seems present in the slides in about the proper 

 quantity to account for the carbon shown by the analysis and may 

 be graphitoid, or some other carbonaceous residue, instead of 

 graphite. 



All the slides contain much calcite. It fills the amygdules, it 

 occurs in irregular patches throughout the balls, and it solidly 

 wxlds up the numerous cracks which run everywhere through 

 the lava. 



The calcite in the cracks gives clear evidence of the deformation 

 to which the rock has been subjected since it solidified. It shows 

 everywhere the close-set, multiple twinning, and the undulatory 

 extinction produced by the deforming stresses. 



At the borders of the limestone inclusions in the balls a slight 

 chilling effect is manifest, and a narrow zone of glass containing 

 much finely divided, black, opaque matter has developed. A slight 

 amount of corrosion has also occurred, small fragments of the 

 limestone appearing separated from the main mass and in all 

 stages of solution in the lava, the resulting product being a clear, 

 light green glass. Examples of partially corroded limestone sur- 

 rounded by such glass are shown, together with others which 

 suggest the utter disappearance of the limestone fragments, the 

 glass indicating their original presence. The limestone has also 

 been recrystallized by the action of the lava, the border of the 

 inclusion being more coarsely crystalline than the interior. 



The pitchstone which constitutes the intervening material is 

 black, but becomes a clear, light green glass in thin section. It is 

 perfectly clear and unaltered, a matter of some surprise when it 

 is recalled how completely the olivines are altered. Except in the 

 vicinity of limestone inclusions, it has been sheared and shows 

 its condition of strain by being doubly refracting. It is locally 



