k JAN 4 - 1922 *! 



13 



Alnoitic Rocks at fide Cadieucc, Que. 



the replacing minerals principal^ biulileTinonticellite 

 and melilite. 



There is no very obvious tendency for any one replac- 

 ing mineral to be substituted for any particular original 

 mineral. The replacement is rather of the mass as a 

 whole. Occasionally, however, a more or less definite 

 pseudomorph of monticellite after augite may be noted 

 and frequently a monticellite band is interposed between 

 mica and augite. Moreover, in facies of the rock where 

 augite was absent monticellite is not developed. The 

 evidence is good that augite is the principal if not the 

 sole source of the monticellite. (See Plate 1(d).) 



The original augites were large crystals approaching 

 1 cm. in diameter but the corroded remnants are seldom 

 one-tenth as large. Several adjacent individual remnants 

 embedded in the replacing minerals are frequently seen 

 to be similarly oriented, with their cleavages and extinc- 

 tions coincident, and it is not difficult to picture the 

 original augites now largely replaced. (See Plate 1(b).) 

 Where the replacing mineral is biotite it is practically 

 always paler in color adjacent to augite. 



The original chrysolites were rather less than one- 

 half as large as the augites. Three or four adjacent rem- 

 nants can frequently be connected up to construct the 

 original crystals, though this is not so conspicuous as 

 in the case of the augites. Whether biotite, monticellite 

 or melilite is the replacing mineral the chrysolites nearly 

 always show a rim about them of particularly strong 

 concentration of ore minerals. (See Plate 1(c).) The 

 augites occasionally show such a rim but it is usually 

 within the border rather than at the border. There is a 

 development of very fine crystals of perovskite in these 

 rims and as one passes outward coarser and coarser 

 crystals are encountered. They are plentiful in the mon- 

 ticellite, melilite and biotite, but the central portions of 

 chrysolite and augite crystals are free from them. There 

 is a strong suggestion, then, that the perovskites were 

 formed as a result of reaction upon the original minerals. 

 As has been noted before, where monticellite resorbs and 

 replaces chrysolite the two are frequently in optical con- 

 tinuity, their boundary being marked by the dark band 

 of ore minerals. 



WTien one thus separates the minerals into the two 



