MUSEUM OF COMPARATIVE ZOOLOGY. 237 



the long crystals. I'owder feebly magnetic. — Section. Much decoin- 

 posed. The feldspars retain their outline, but arc lilled with chloritic 

 material, — kaolin, epidote, and calcite. IMagnctitc is very plentiful in 

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

 face (leucoxene), which, in connection with the reticular or branching 

 shape of the masses, shows the presence of raenaccanite. Pyritc 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, epidote, 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 Exposure of the Dike in the Field midway hetiveen the 

 extreme Eastern and Western Ridges. 



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

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

 fresher rock than those already described. The few porphyritic feld- 

 spars are generally plagioclase, and exhibit the same proof of an early 

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

 flow around them, etc.). The feldspars of the groundmass are principally 

 plagioclase, but some Carlsbad twins and unstriated crystals can be 

 found. All these feldspars are compai'atively fresh, and the formation 

 of the greenish scales and other products of decomposition has not pro- 

 gressed far. The frequent inclusions of the original base, however, ai'e 

 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 quartz. Chloritic products and magnetite represent the 

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

 rare. True amygdules occur, filled with chlorite, quartz, and epidote. 



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

 and not in Line ivith the Western Jlalf, though trending parallel ivith it. 



Lens. A reddish groundmass, containing feldspar crystals, amyg- 

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



