PETROLOGY 325 



individuals with crystal faces on the inner border, although in some 

 instances complete forms occur (see figure 2). 



The analcite is sometimes faintly doubly refracting in parts of the in- 

 dividual. Distinct traces of cubic cleavage sometimes occur. Replace- 

 ment of analcite by calcite and fibrous zeolitic material is commonly ob- 

 served, and in many instances only a small percentage of the analcite 

 remains. In some cases the alteration has progressed first from the inner 

 zone, in others from the outer zone, and sometimes from both. That the 

 analcite is a pyrogenetic constituent of the rock is practically certain. 

 Its occurrence indicates that two periods of crystallization are repre- 

 sented, the first that of material which had segregated in the magma 

 resulting in the formation of a single individual or a group of individuals, 

 and a second of a later period developing the irregular grains in the 

 ground-mass. 



Biotite occurs in minor quantity as small individuals which sometimes 

 show crystal form. Basal sections are dark reddish brown. Sections 

 normal to the cleavage are pleochroic, ranging from very dark brown to 

 light yellow. It is usually fresh, but sometimes shows alteration to 

 chlorite. 



Magnetite, titanite, rutile, apatite, and quartz occur as original minor 

 accessory constituents. Magnetite is abundant as inclusions in the younger 

 minerals and is usually idiomorphic in outline. Titanite is developed in 

 small anhedra, with sometimes minute inclusions of rutile crystals, bni is 

 not abundant. Apatite as small euhedral and rounded forms occurs as 

 included crystals in the other minerals and as separate larger idiomorphii 

 crystals, measuring up to 0.5 millimeter in diameter among the minerals 

 of the ground-mass. Quartz does not occur among the minerals of the 

 ■ground-mass, but only within the hornblende, where il is developed as 

 small inclusions of microscopic size and larger masses as the core ol' a 

 hornblende crystal, constituting as much as 25 per cent of ils volume. 



The secondary minerals are zeolites, biotite, chlorite, calcite. pyrite, 

 and a. small amount of epidote. The zeolites occupy areas within the 

 altered poet ions of the analcite and plagioclase feldspar. Biotite occurs 

 in small flakes and is associated with the zeolites and epidote. Calcite 

 occurs in the same manner as a replacement of the various lime bearing 

 minerals in (lie t liin-sect ions. Chlorite is most abundant in areas occu- 

 pied originally by augite, and it also occurs as an alteration product of 

 biotite and to a small extent of hornblende. Pyrite IS found associated 

 with chlorite, magnetite, and calcite in partially altered augite. 



Chemical composition <ni<l classification in the quantitative system. 

 The composition of this POCk Is shown in the chemical analysis given in 



column I of the subjoined table, and there are added for comparison 



