66 THE ROYAL SOCIETY OF CANADA 



composition of the principal type, and megascopicaliy the rocks 

 are similar in appearance. 



The first minerals to crystallize were, as before, brookite, perov- 

 skite, and black iron ore. The iron ore is in rather irregular grains, 

 but the brookite and perovskite, as described above, crystallized in 

 minute squares and octahedra. 



Regular idiomorphic crystals of chrysolite and colourless augite 

 are present, the former in relatively larger amount than the latter. 

 Chrysolite often occurs as rounded grains enclosed in larger biotite 

 crystals. 



Biotite occurs in hypidiomorphic individuals, usually with one 

 or more straight edges, and is lighter in colour than in the type de- 

 scribed above. Inclusions of all the above accessory and idiomorphic 

 minerals are common, and the contact between biotite and monti- 

 cellite is usually characterized by straight edges. This mineral is 

 still the most abundant constituent of the rock, but does not exceed 

 all the other constituents as it does in the principal type. The poiki- 

 litic variety is not nearly so prominent nor are the separate individuals 

 in close contact as a rule. 



Monticellite usually occurs in hypidiomorphic prisms, although 

 the majority of the crystals of chrysolite have a very narrow border 

 of a mineral with lower birefringence which was taken to be monticel- 

 lite. 



Besides the above regular forms, there is a coarse groundmass 

 consisting of monticellite and carbonate. 



The monticellite present in the groundmass forms irregular 

 fragments filling the interstices between biotite and other minerals 

 such as chrysolite, and, from its relations with the biotite, it appears 

 as if it had begun to form before the formation of the biotite and had 

 continued after the latter had been completely crystallized from the 

 magma. 



Calcite also occurs in the groundmass in irregular masses and 

 is very probably a product of hydrothermal action. Veinlets of 

 calcite, such as are described above, are lacking here, but as the 

 olivine is extremely fresh except for the border of monticellite, indi- 

 cating slight resorption, it is difficult, if not impossible, to dismiss the 

 calcite simply as a product of weathering. 



The minor differences between this type and the principal type 

 appear to be due to a lesser degree of resorption, possibly owing to 

 slightly more rapid cooling. 



Notes on the Occurrence of Calcite. — ^Although there is a great 

 deal of calcite in all the sections examined from this area, no 

 trace of the partial carbonatization of any primary constituent was 



