Alnoitic Rocks at Isle Cadieux, Que. 3 



is not highly colored, its strongest absorption being # a 

 rather pale greenish brown. In this respect it is, 

 however, markedly variable in a single crystal and in 

 particular tends to be less strongly colored where it abnts 

 against enclosed angite and more strongly colored where 

 it is against chrysolite. Its refractive indices are 

 approximately 7 = 1.61, a = 1.56 and it is nearly uniaxial. 



M'onticelliie is, on the whole, the second constituent 

 in point of abundance. It occurs in fairly large grains 

 up to 1.5 mm. in diameter. These likewise include 

 corroded remnants of chrysolite and augite crystals, 

 but since the monticellite is not in such large grains 

 as the biotite, the pcikilitic nature is not so obvious. 

 Occasionally the monticellite is in optical continuity 

 with an enclosed chrysolite grain. The optical prop- 

 erties of the lime-magnesia olivine, monticellite, agree 

 with those of the same mineral as described by Pen- 

 field and Forbes from Magnet Cove. 6 The birefrin- 

 gence is moderate, the colors in thin section rising only 

 to an orange-red of the first order and usually falling far 

 short of that color. The optic axial angle 2V = 70° ± 5° 

 and the sign is negative. 7 In a section normal to the 

 optic axis there is a marked bending of the bar and the 

 sign is readily determined. In these respects the mineral 

 is distinct from the other olivine, chrysolite, which fre- 

 quently shows the brilliant colors of higher orders and 

 for which the axial bar remains so nearly straight that 

 one cannot be sure of the direction of its curvature or of 

 the optical sign. The difference of birefringence is 

 apparent in this interference figure also, for the chrysolite 

 shows the inner colored lemniscates, whereas these are 

 beyond the aperture of the objective in monticellite. 



It is, however, particularly in their quite different 

 relations to the other minerals that one is led in the first 

 instance to suspect the existence of two distinct olivines 

 in the rock, the monticellite occurring as oikocrysts, the 

 chrysolite as chadacrysts. 8 These relations will be 

 described in detail later, but advantage was taken of the 



6 This Journal, (4) 1, 135, 1896. 



7 The value of 2V for monticellite from Magnet Cove has been incorrectly 

 transcribed into the texts of Iddings, Winchell and Rosenbusch, where it is 

 given as 37°31'. This is really the value of V as determined by Penfield and 

 Forbes. 



8 Iddings, Igneous Rocks, I, p. 202, 1909. 



