254 J. F. KEMP — ROCKS OF THE EASTERN ADIRONDACKS. 



stances the unchanged core of the serpentine has been found to be an 

 isotropic mineral of a high index of refraction. It gave no interference 

 figure, and must therefore be isometric. It is probably a garnet, for 

 spinel is practically the only other isometric mineral which would be 

 likely to occur in these surroundings, and the great resistance of spinel 

 to alteration makes it improbable that it would be the center of a ring of 

 decomposition products, especially when serpentinous or chloritic in 

 character. 



The ophicalcite ledges opened in the quarries have many knots or 

 small lenses of silicates, analogous to those in the white limestone, and 

 they yield beautiful crystals when the calcite is dissolved in acid. The 

 best and most interesting are white or slightly pink transparent pyrox- 

 enes, an unusual variety, now being investigated by H. Ries,. An 

 analysis by Mr Ries yielded the results in column I, and in column II 

 there is given for comparison an analysis of Port Henry material by Mr 

 G. P. Merrill.* 



I. II. 



SiO., 54.57 55.36 



Fe^Os 0.22 



FeO 1.62 0.57 



AlA 1.32 0.22 



CaO 23.25 24.48 



MgO 17.78 19.53 



K2O 0.70 



H2O 0.32 



99.56 100.38 



Brown hornblendes, with terminations, light yellow titanite and 

 phlogopite make up the remainder, and the last-named occur in bunches 

 but slightly mixed with other minerals. 



The silicate inclusions are drawn out in lenses, according to the pressure 

 to which they have been subjected, and give a coarse flow-structure to 

 the quarry-face. 



The hornblendic and other Schists and Gneisses. — These rocks invari- 

 ably accompany the limestone, and with their dark, rusty outcrops con- 

 stitute in many respects the most prominent member of the series. The 

 structure is often quite gneissic, being sufficiently coarsely banded to 

 warrant this term. Feldspar laminations may appear and heighten the 

 contrast, but the commoner form is that of a tj^pical coarse hornblende- 

 schist. Under the microscope brown hornblende is seen to be the most 

 abundant mineral. It occurs in large, irregular crystals and smaller 



*See Proceedings U. S. National Museum, vol. xii, p. 596. 



