[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 85 



The augite occurring in these aggregates is similar to that in the 

 main part of the rock except that the grains are rarely over 0.02 mm. 

 in diameter. 



Aegirite-augite is green in colour and is slightly pleochroic in 

 grass green to bluish green shades. The crystals do not attain a 

 diameter of more than 0.2 mm. and show a tendency to zoning. The 

 mineral usually occurs as a border of intergrown crystals, often con- 

 tinuous about these patches. It does not occur elsewhere in the rock, 

 and is not present to any extent in those patches where microcrystal- 

 line augite is present. The extinction angle is about 35°. 



Heulandite occurs most frequently in those areas in which 

 aegirite-augite is found, but is also present in small amounts in the 

 areas filled with minute augite crystals. It forms small laths in the 

 glass, and is colourless with a low birefringence and a refractive index 

 only slightly higher than that of the base. It is biaxial and negative, 

 and the cleavage is very indistinct. 



The feldspar is more basic than the variety present in the main 

 part of the rock, and is negative in character. It is considered to be 

 bytownite about Abi An4. 



Calcite is present in irregular grains, and although it is present 

 in small amount in the rest of the rock, by far the greatest amount is 

 associated with these included areas. 



The isotropic base present in these patches is similar to that in the 

 groundmass of the remainder of the rock. 



The occurrence of these patches is possibly due to the segregation 

 of part of the residual magma after the crystallization of the amphi- 

 boles, and biotite, and probably the greater part of the augite. This 

 residue was rich in lime, alkalies, and silica, and probably contained 

 a large part of the more volatile constituents of the magma such as 

 water and carbon dioxide. Elsewhere the magma was not held in 

 confinement and the normal sequence of crystallization was followed ; 

 labradorite followed the ferro-magnesian constituents, and was in 

 turn succeeded by glass. In these areas which were in some way shut 

 off from the rest of the magma, the order of crystallization was either 

 of the normal type as in the places where microcrystalline augite 

 occurs, or else some of the soda which otherwise would have entered 

 the labradorite was included in the augite forming aegirite-augite, and 

 the remainder of the magma crystallized as bytownite, heulandite, 

 calcite, and the isotropic base. 



Although these patches have been assumed above to be segrega- 

 tions within the magma, it is quite possible that they were injected 

 into the rock along with the later intrusion. The contact between the 

 two phases is sharp, and phenocrysts protruding from the rock into 



