[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 



63 



aa Very abundant 



a Abundant 



b Subordinate 



c Present 



Principal or Porphyritic Type. — ■ The principal or porphyritic 

 type whose megascopical features are described above, is found to 

 consist of phenocrysts of augite, chrysolite, and biotite in a ground- 

 mass of monticellite, biotite, and calcite, with brookite, perovskite, 

 apatite, and opaque iron ore as accessory constituents. 



Augite is present in very small amount and is of the colourless 

 variety. It occurs as regular idiomorphic crystals, up to 0.2 mm. in 

 length, and has undergone only a slight amount of alteration along 

 cracks. The extinction mounts to 45°. 



Chrysolite is an abundant constituent of the rock, and under 

 ordinary light has, as a rule, regular idiomorphic outlines. The 

 crystals are about 0.3 mm. in length and are usually prismatic and 

 terminated at each end by a dome which may or may not be truncated 

 by a basal pinacoid. Under crossed niçois, it is found that frequently 

 the chrysolite occupies only about fifty per cent of the grain, while 

 the exterior is composed of monticellite. Inclusions of iron ore, 

 perovskite, and brookite are common. The chrysolite is frequently 

 enclosed by biotite, and here the monticellite border is lacking or 



^Calcite occurs in small veinlets. 



^The alteration to monticellite is less strongly marked in this phase. 



