MUSEUM OF COMPARATIVE ZOOLOGY. 



237 



the long crystals. Powder feebly magnetic. — Section. Mnch decom- 

 posed. The feldspars retain their outline, but are liiled with chloritic 

 niaiei-ialj — kaolin, epidote, and calcite. Magnetite is very plentiful in 

 crystalline and irregular forms, having often a whitish, decomposed sur- 

 face (leucoxcno), wliich, in connection with the reticular or branching 

 shape of the masses, shows the presence of mcnaccanite. Pyrite occurs 

 in occasional grains and square crystals, generally close to or mingled 

 with the magnetite or decomposed menaccanite, and is therefore proba- 

 bly an alteration product. The remaining portion of the rock is a con- 

 fused mixture of chlorite, cpidote, quartz, viridite, hornblende, calcite, 

 and colorless needles, in part probably apatite, — all products of altera- 

 tion. This rock is the most coarsely crystalline and the most decom- 

 posed of any examined. 



[7.] From the Exposicre of the Dike in the Field midway hetweeu the 



extreme Eastern and Western Ridges. 



Lens. Similar to the preceding hand-specimens, but rather red- 

 dish in color, and somewhat more amvirdaloidal. — Section. A much 

 fresher rock thiui' those already described. The few porphyritic feld- 

 spars are generally pLagioclase, and cxliibit the same proof of an early 

 crystallization mentioned above (i. c. the feldspars of the groundmass 

 flow around them, etc.). Tlie feldspars of tlie groundmass are principally 

 plagioclase, but some Carlsbad twins and unstriated crystals can be 

 fovuid. All these feldspars arc comparatively fresh, and tbo formation 

 of tlie greenish scales and other products of decomposition hlis not pro- 

 gressed far. Tho frequent inclusions of the original base, however, are 

 entirely altered to chloritic products and magnetite. The feldspars con- 

 tain occasional large rounded or irregular fluid inclusions, with bubbles, 

 and immense numbers of extremely small similar inclusions (requiring 

 the use of powers of from 700 to 900) characterized also by occasional 

 moving bubbles. Grains and crystals of epidote occur in the feldspars, 

 and occasionally (piartz. Chloritic products and magnetite represent the 

 original base. Epidote occurs in the groundmass in patches ; calcite is 

 rare. True amygdulcs otjcur, fllled with chlorite, quartz, and epidote. 



[8.] From the Ridge constituting the extreme easterly Exposure of the Dike, 

 mid not in Line with the Western Half, though trending parallel with it. 



I^ens. A reddish groundmass, containing feldspar crystals, amyg- 

 dules of greenish chlorite, and red spots resulting from the dccomposi- 



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