32 



It is a well known fact that we have in the felspars of the plagioclase 

 group a series of forms intermediate between albite and anorthite (1) and that 

 the physical characters, of which the optical are the most important for our 

 present purpose, are definitely related to the chemical so that conclusions 

 as to the latter may be drawn from an examination of the former. Now, in 

 the porphyritic felspars of such rocks as the andesites, we constantly find 

 crystals, built up of successive zones of material, having different optical 

 characters. Thus, in the large felspars of the Cockfield and Ayton Dyke, 

 which must, on the whole, be referred to labradorite, we frequently find an 

 external zone having optical characters more nearly allied to bytownite. (2) 

 Sometimes three or four zones may be observed exhibiting optical characters, 

 which do not indicate a variation in any one definite direction ; a zone of 

 material allied to bytownite or anorthite, for instance, may be intercolated 

 between two, having the characters of labradorite. (8) 



The observations of HOPFNER (4) and BECKE (5) appear to lead to the 

 conclusion that in the majority of cases the felspar of the central portion is 

 more basic in composition, that is more allied to anorthite, than that of the 

 exterior. Sometimes the successive zones are divided from each other by 

 tolerably sharp lines, but very commonly there is a perfectly gradual transi- 

 tion. Fig. 16 illustrates this feature. 



Fig. 16. (After Becke.) 



The position of the nicols is indicated by the cross. The central portions 

 of the principal individual are light ; the corresponding portions of the twin 

 lamellae are dark. In the exterior portions the relations of the twin lamellae 

 and principal individual are exactly reversed ; and there is a gradual transition 

 from one condition to the other. 



Zonal structure is also common in many other minerals especially augite, 

 hornblende, and tourmaline. In the cases of the last-mentioned minerals it is 

 usually indicated by variations in colour. 



(1) Ifber die optische Orientirung der Plagioclase. Max Schuster T.M.M., 1881, 

 vol. III. p. 117. 



(2) TEALL. Notes on some North-of -England Dykes. Q.J.G.S., 1884, p. 217. 



(3) M. LEVY has endeavoured to explain these variations in optical character without assum- 

 ing variations in chemical composition. Comptes Rendus, 23rd Jan., 1882. 



(4) N.J., 1881, Vol. II., p. 183. 



(5) T.M.M., Neue Folge, Vol. V., p. 161. 



