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



The augite is of a bright green, with a faint pleochroism to yellow. It 

 is often intimately associated with garnet, so much so that the same 

 cracks pass through both minerals and they almost shade into each 

 other. It is pleochroic — yellow a, green t and c. An optic axis emerges 

 nearly at right angles to the base, and the plane of the optic axes is the 

 plane of symmetry. 



The garnet is a good red — fairly intense. It occasionally displays the 

 rhombic dodecahedron, but is mostly irregular. No optical anomalies 

 were noted. A little apatite is the only other mineral in the micro- 

 section. 



The Limestone. — The limestone bands at Keene are thickly charged 

 with silicates and with magnetite. The former affords a pyroxenic lime- 

 stone not serpentinized in all cases. The limestone * also contains, in 

 addition to pyroxene, abundant cinnamon-colored garnets of rude crys- 

 tallographic outlines which may reach an inch and a half in diameter. 

 They are seldom pure garnet, but have green pyroxene mixed all through 

 them. This association of garnet and pyroxene, which is so extremely 

 common all through the region of the Norian rocks, needs chemical inves- 

 tigation. There is probably no great difference in the elements present 

 in the two, and indeed a comparatively slight molecular rearrangement 

 of either — an addition of bases or a loss of silica — would bring about the 

 passage of the bisilicate into the unisilicate. 



Some interesting parallel cases have been observed abroad, but as the 

 question is one of mineralogic interest rather than of broad geologic im- 

 portance, it has not yet been investigated. The crystallographic faces of 

 the garnet are pitted and seamed. There are also granular masses of the 

 same mineral, and the larger garnets contain magnetite and calcite as 

 inclusions. 



The ore is a portion of the limestone especially rich in magnetite, and 

 greenish yellow pyroxene almost always accompanies it. Less common 

 specimens are formed of a mass of magnetite and most intimately in- 

 volved biotite. The ore body proved to be a good sized one at the Hale 

 or Weston mine, the place where the section is taken. The Tenth 

 Census reported that it is an irregular mass in the limestone, which is 

 worked from 200 to 300 feet in depth ; that it is from 8 to 16 feet thick, 

 dips at a high angle to the northwest, pitches at forty-five degrees to the 

 northeast, and that it is in white crystalline limestone, much of which 

 was mixed with the ore. 



* Selected limestone free from included minerals has been analyzed by H. Ries at the writer's 

 request and the results are as follows: H2O, 2.16; Fe203, A.I2O3, 1.72 ; CaO, 46.79 ; MgO, 5.145; CO2 

 (calculated for CaO and MgO), 42.42; total, 98.235. The limestone was probably an original siliceous 

 dolomite, the silica and much of the magnesia of which are now segregated in the disseminated 

 pyroxenes. While the writer has had before him the possibility of these ophicalcites being ex- 

 cessively altered gabbros or peridotites, the other view is regarded as more reasonable. 



