[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 57 



Frequently the chrysolite occurs in small grp.ins surrounded by 

 a rim of monticellite, and the whole is in turn embedded in a carbonate 

 which is probably calcite; the inner part, however, is quite fresh. 



Some idiomorphic crystals of chrysolite were also observed 

 attaining dimensions of 0.5 by 1.0 mm. One was found abutting 

 against a fragment of unresorbed biotite, and was deformed at the 

 other end by a fragment of chrysolite which had partially altered to 

 monticellite. These individuals probably represent a second genera- 

 tion of olivine. 



Monticellite occurs, as a rule, in irregular grains up to 1.5 mm. 

 in diameter as well as rims about the chrysolite. Its outstanding 

 characteristics have been described above. 



Melilite is an important constituent of the rock, forming a large 

 part of the groundmass. It occurs in laths 0.2 by 1.0 mm. in size. 

 These laths occasionally exhibit the characteristic peg structure of 

 melilite and are, as a rule, negative in character. The basal cleavage 

 is quite distinct in some individuals. Larger individual grains of 

 melilite also occur. The mineral is closely associated with calcite 

 and frequently is entirely surrounded by the latter mineral. The 

 melilite itself, however, appears to be quite fresh. 



Augite is comparatively rare in the rock, only a few scattered 

 grains are found. 



Black iron ore occurs in regular crystals and irregular grains and 

 is included in all the older minerals. Part appears to be primary 

 wKile a great deal has been derived from the alteration of augite and 

 biotite. Many grains apparently contain a centre of pyrite, and 

 pyrite occasionally occurs along minute cracks. The grains have a 

 maximum diameter of 0.2 mm. 



Perovskite is present in considerable amount and usually occurs 

 in minute squares or octahedra. Like the iron ore, it is included in 

 all the other minerals but less frequently iji the minerals of the older 

 generation, namely, the chrysolite and augite. 



Apatite is present in minute laths and hexagonal sections. 



The carbonates which are mainly calcite have been referred to 

 above and consist of irregular grains surrounding mineral grains or 

 filling interstitial spaces. They are very seldom associated with the 

 earlier minerals, but, as has been noted above, frequently surround 

 melilite and monticellite, forming with the former the bulk of the 

 groundmass. It may be supposed that the carbon dioxide was 

 introduced by ascending solutions after the final solidification of the 

 rock. The occurrence of calcite in these basic rocks will be discussed 



