[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 73 



A third mineral which exhibits to a certain extent the same 

 alteration phenomena is biotite. This mineral is comparatively light 

 in colour, ranging from a pale yellowish brown to light brown. The 

 "biotite occurs in occasional large plates up to half an inch across 

 surrounded by a dens;e border of magnetite grains. The forms are 

 much corroded, and several instances show the reaction working 

 into the heart of the crystal, the reaction being favoured by the 

 strong cleavage of the mica." In the sections examined by the writer, 

 although most of the individuals show this alteration, some are 

 surrounded by a semi-opaque rim similar to those around the oliyine 

 and augite. Still others show no alteration about the edges and 

 occur in very irregular jagged grains. The biotite in the gro'undmass 

 is occasionally in optical continuity with neighbouring larger indi- 

 viduals, and it is suggested that these unaltered fragments were 

 originally much larger, and were enclosed in a rim of magnetite and 

 possibly also the semi-opaque material, but that this border together 

 with some of the unaltered mica had been torn away, and that the 

 fragments thus removed were scattered through the still liquid 

 magma and, on the final solidification, formed a part of the ground- 

 mass. 



Rounded grains consisting entirely of iron ore and the semi- 

 opaque rim are common, as well as many grains of the semi-opaque 

 material without the iron ore. It is thought thiat the former represent 

 completely altered biotitfes, although it is possible that ilmenite 

 was the primary mineral in which some trace of the cleavage still 

 remains, while the latter are either wholly metamorphosed olivine or 

 augite. 



Other minerals present as phenocrysts are apatite, in irregular 

 rounded grains, and small amounts of labradorite which is largely 

 altered to calcite. 



Niatrolite occurs filling amygdules in places. 



The groundmass is composed of a granular aggregate mainly of 

 iron ore, biotite, and serpentine. Perovskite in minute grains is 

 abundant. 



The earlier minerals such as olivine, augite, and biotite appear to 

 have formed while the magma was rising and their formation occupied 

 a considerable period as the phenocrysts are quite large. After this 

 partial crystallization conditions changed, the phenocrysts were 

 partially resorbed in the residual magma, and in some way parts 

 of the resorption rims and even of the unaltered minerals were re- 

 moved by abrasion and scattered through the groundmass. It also 

 appears as if the rock comprising this main intrusion represents 



