[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 67 



noted, although irregular grains of calcite are frequently included 

 in the hypidiomorphic grains of biotite. It cannot be assumed that 

 in every case, where carbonatization has begun, it has been carried 

 to completion and no trace of the original mineral can be found, 

 excepting in these biotite grains. Chrysolite, which is often closely 

 associated with the calcite, is quite fresh. Calcite was never defi- 

 nitely recognized as a pseudomorph and, where present, always 

 occurs as irregular grains with inclusions of perovskite, brookite, and 

 iron ore, and frequently encloses idiomorphic olivines which have a 

 narrow border of monticellite, but show no other form of alteration. 

 It, therefore, seems necessary to find some explanation for the presence 

 of calcite other than to state that it is simply a product of weathering. 



Sections cut along the small veinlets consisting of biotite and 

 calcite show a peculiar alteration of the biotite. In. places, radial 

 spherulitic aggregates of fibres occur which, in ordinary light, are 

 colourless, and have a lower refractive index than the biotite. With 

 crossed niçois they have a low birefringence, and possess the aggre- 

 gate polarization of chlorite. All variations are seen, from the in- 

 cipient fibrous material which has the same colour as the mica and 

 is not distinguishable from the biotite in ordinary light, to small 

 areas having an idiomorphic outline, and composed of small aggre- 

 gates of chlorite. Some of these areas, however, contain calcite in 

 place of chlorite. These areas occur along the plane of symmetry 

 of the biotite and appear to have been originally cavities in the mica. 



Thus, after the solidification of the rock, cracks were formed 

 and these cracks were filled with biotite and calcite. Later ascending 

 solutions partially altered the biotite to chlorite and any cavities re- 

 maining were filled with calcite. Whether the calcite found through- 

 out the rock is due to hydrothermal action or is primary, it is im- 

 possible to say. 



Fine-Grained Variety. — The fine-grained material which is merely 

 a textural variant of the principal type is more equigranular owing to 

 the absence of poikilitic biotite, but otherwise is very similar in 

 mineralogical composition. 



Chrysolite is by far the most abundant constituent, and almost 

 invariably occurs as regular prisms terminated by domes. These 

 crystals are usually not more than 0.5 mm. in length, and are, as is 

 usual in these rocks, partially replaced by monticellite about the 

 borders. 



Augite is also present in idiomorphic individuals but to a smaller 

 extent than the chrysolite. 



As before, monticellite occurs as hypidiomorphic grains, as 

 resorption rims about the chrysolite, and also in the groundmass. 



