part 4f\ 



PETROLOGY OF THE AltNAGE DISTRICT. 



4o5 



then, in the Arnage case, cemented by black iron-oxide. The hyper- 

 sthene alters into pale-green fibrous hornblende. The times of 

 finishing of crystallization of hypersthene and felspar appear to be 

 about the same, neither mineral being strictly euhedral. 



In some slices diallage occurs in small amount, perhaps two 

 plates being seen in a slice of average dimensions. It usually 

 forms large, much-schillerized, ophitic plates enclosing small felspars 

 and hypersthene, and is edged by hornblende. 



The hornblende is pleochroic as follows : — a pale yellow-green, 

 b deep brown, c green-brown, with the absorption c = i)>a, 

 and C : c = about 10°. It forms an edging to hypersthene or 

 diallage, or builds larger plates with hypersthene kernels, these 



Fig. 3. — Norite of the sheet. 



^ 



M 



A=Labradorite (Lab.) and hypersthene (Hy.), with scarce biotite (Bi.), quartz 



(Qz.), and iron-oxide. X 23 (see p. 454). 

 B = Illustration of the reaction series; hypersthene (Hy.), hornblende (Ho.), 



biotite (Bi.), and quartz (Qz.). Solid black represents iron-oxide. 



X 46 (see p. 456). 

 C = Illustration of the reaction series; olivine (01.), hypersthene (Hy.), 



hornblende (Ho.), and biotite (Bi.). X 46 (see p. 456). 



kernels having a ragged edge against the enclosing hornblende 

 (see fig. 3). In its turn, too, the hornblende is often bordered by 

 yellow to deep-red biotite, with which are associated large 

 irregular grains of black iron-oxide. Micropegmatitic inter- 

 growths of hornblende, biotite, and quartz are noted (see fig. 3). 



Apatite occurs in accessory prisms which sometimes reach 

 1 mm. in length ; black iron-oxide is always present, and usually 

 associated with the biotite. 



Q. J. G. S. No. 31G. 2 i 



