234 



BULLETIN OF THE 



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[l.J From Western End of Dike ^ North Side, near the Road, 



small Dikes in the Amygdaloid. 



one of the 



Lens.^ A greenish-gray, fclty-looking rock, containing minnte grains 

 of pjrite, and small feldspar crystals. Traversed by vcinlcts of epidotc. 

 — /Section, White opaque feldspar crystals, and masses of opacite, niug- 

 nctlte, and pyrite, in a green chloritic groundmass. The feldspars have 

 generally the long ledge form of the basaltic triclinic feldspars, but 

 occasionally the form of Carlsbad twins of sanidin. They are entirely 

 altered to a fibrous and sc^i,ly aggregate, polarizing with yellow and blue 

 colors, — often with the brilliancy of talc. Colorless needles with cross 

 fracture (apatite) occur occasionally in the feldspars, and also aggregate 

 quartz. Between the feldspars lies a mass of green fibrous products, — ■ 



chlorite, viridito, etc., considerable epidotc, magnetite, quartz, etc., 

 rarely hematite and biotite. The magnetite often has the form of a 

 grating, reminding us of decomposed olivine. The feldspars occasion- 

 ally have a :f!uidal arrangement. 



[2.] Contact of Amygdaloi/l and GongUnnerate at Southeast Corner. 



Section. A mass of small feldspar crystals, having a well-marked 

 fluidal arrangement, and surrounding decomposed crystals of olivine and 

 masses of magnetite and opacite. The olivine crystals have the charac- 

 teristic lozenge shape, blackened border, and irregular fissuring, while 

 the small parallel feldspars of the groundmass separate and How around 

 the crystals. Some are altered within the black border to a light green 

 serpentine with fibrous polarization ; in others, wliile the centre shows 

 the brilliant polarization and the pitted surface of olivine (though the 

 greenish color is evidence of some alteration), the exterior zone of the 

 crystal has been altered to a bluish-gray substance, vdiich in polai'ized 

 light is seen to contain fibres with brilliant polarization, and msiy per- 

 haps represent a stage in the alteration to talc. Some of the olivines 

 are wholly or partially altered to ferrite and talc, the latter polarizing 

 very brilliantly. Some of the magnetite and opacite in the section is 

 derived apparently from the complete alteration of grains of olivine. 

 The feldspars have generally the long ledge character of the l)asaltic 

 feldspars, though some have the form and optical properties of Carlsbad 

 twins. Occasionally there is found a crystal sufiiciently fresh to show 

 the multiple twinning, but generally they are filled with greenish or 

 transparent scales, while along the centre of the ledge crystal there 

 runs a line of green chloritic material, containing generally less opacite 



