[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 65 



The biotite is deep brown in colour and is strongly pleochroic, 

 while the bleached border so noticeable in the Como occurrence is, 

 to a great extent, lacking. The mineral appears to contain more 

 iron than the Como variety as it is much deeper in colour. 



Iron ore, perovskite, and brookite occur in minute crystals, the 

 last two usually idiomorphic. The titanium oxide present is a light 

 wine red in colour and occurs in minute squares. As the grains were 

 too small to show an interference figure, or indeed to show even the 

 order of the birefringence, its identification as brookite is somewhat 

 uncertain. It has, however, a very high refractive index and bire- 

 fringence and no apparent cleavage. These facts, together with its 

 occurrence in squares, led to its identification as brookite, since rutile 

 and anatase, with which it might be confused, have altogether different 

 crystal habit. 



Perovskite occurs in the typical wine yellow squares and octa- 

 hedra with high refractive index and is isotropic. 



Calcite is not present in large quantities except along the tiny 

 veinlets noted above. In the fresh rock it occurs in very small grains, 

 usually within monticellite or in the hypidiomorphic biotite crystals 

 where it frequently occupies about fifty per cent of the grain and is 

 situated at the centre. The minerals in which it occurs are otherwise 

 quite fresh and there is practically no trace of serpentinization of the 

 olivine or other products of weathering. The nature of this calcite 

 is described more fully below, as its development is more clearly shown 

 in other parts of the rock. 



The order of crystallization of the rock thus appears to be as 

 follows: After the formation of the iron and titanium accessory 

 minerals, chrysolite and augite, the most basic ferro-magnesian constit- 

 uents of the rock, crystallized out. These minerals became partly 

 resorbed by the residual calcic-alkaline magma and there was also 

 a partial replacement of the chrysolite by the lime olivine, monti- 

 cellite. Thus, the formation of monticellite probably followed the 

 chrysolite very closely. The olivines were subjected to further resorp- 

 tion, while monticellite and the earlier generation of the biotite were 

 crystallizing out. This stage was followed by the final crystallization 

 of the monticellite which appears in the groundmass together with the 

 large poikilitic biotites. Whether the calcite is also a product of this 

 final solidification or not will also be discussed below, but anticipating 

 this discussion a little, it may be stated that, in the opinion of the 

 writer, at least, it was formed by solutions ascending through the 

 magma after it had completely cooled. 



Variations in Principal or Porphyritic Type. — In the southern 

 part of the sheet there are few deviations in the mineralogical 



5— D 



