238 



BULLETIN OF THK 



tion of the rock. — Section. The least decomposed rock of any exam- 

 ined. It has a groundmass composed of small ledge-shaped feldspars, 

 magnetite, chlorite, epidote, etc., enclosing porphyritically a few large 

 feldspars. The majority of the crystals are plagioclase, but there is a 

 considerable number of Carlsbad twins. The small feldspars of the 

 groundmass show the flowing of the base around the large crystals, as 

 described previously. The larger feldspars contain very characteristic 

 inclusions of a base in irregular, reticulated, or cylindrical forms. They 

 often fill a large part of the crystal ; may be zonally arranged ; and are 

 absolutely identical in shape and other characteristics with the inclu- 

 sions of glass or base in the unaltered basalts. These inclusions are now 

 altered to magnetite and greenish chioritic or viriditic products. Be- 

 sides these dark inclusions of base, the feldspars are filled with almost 

 colorless microliths and scales, — the products of the incipient decom- 

 position of the fcldspathic substance, — and very minute fluid inclusions, 

 rounded, cylindrical, or branching. Some epidote and calcito occur in 

 the feldspars. True amygdules arc found, filled with calcite, epidote, 

 and chlorite. Irregular masses of epidote occur as areas of alteration in 

 the groundmass, — the magnetite often in large masses, enclosing the 



u 



small feldspar crystals of the groundmass, and mixed with considerable 

 ferrite. One decomposed crystal may perhaps be referred to olivine. 



[Q. 8'.] 27ie Large Dike running nearly at Right Angles across the Trend 



of the Amygdaloid. 



Lens. A coarse-grained, dark green rock, containing crystals of feld- 

 spar, pyrite, magnetite, and hornblende, in a dark green groundmass. 

 Section. Contains (comparatively speaking) large-sized feldspar crystals ; 

 fibrous, greenish, dichroic hornblende ; crystals of magnetite and pyrite ; 

 decomposed crystals of olivine ; epidote ; and viridite, quartz, apatite, 

 etc. The feldspars are to a great extent kaolinized. The hornblende 

 occurs in irregular masses, shows strong dichroism and brilliant polar- 

 ization, and contains a great deal of epidote in rounded grains. Some 

 of the feldspar crystals lie imbedded in the hornblende, or cross it, just 

 as they do in the case of the augite of the less decomposed diabases, so 

 that this and the whole character of the hornblende indicate that it is 

 (in part at least) a product of the decomposition of the original augite. 

 The olivine occurs generally in shattered crystals, with the usual black- 

 ened border. Tlie interior is altered to greenish serpeutiuous products ; 

 but little spots still show the polarization and other cliaracteristics of 



