486 J. J. H. Teall — Origin of Banded Gneisses. 



the length of the strip, i.e. in the direction of stretching. The idea 

 of a strain ellipsoid is of course only applicable to those masses or 

 portions of a mass which have been subjected to a uniform strain.^ 

 If the different coloured clays yield differently to the deforming 

 stresses the phenomena will be highly complex. 



Now before we are justified in looking favourably on the theory 

 I am endeavouring to illustrate, it is necessary to show, (1) that 

 banded gneisses are on the whole identical with plutonic rocks in 

 composition, (2) that masses of plutonic rocks are often heterogeneous, 

 and (3) that heterogeneous masses, if such exist, may be deformed 

 in the manner required to produce banded structures. 



To establish the first point it is only necessary to refer to Eoth's 

 elaborate collection of rock analyses.^ No one who has studied these 

 analyses can doubt for a moment that the banded gneisses are 

 essentially identical with plutonic rocks so far as chemical composi- 

 tion is concerned. The resemblance, however, is not limited to 

 chemical composition, it extends to mineralogical composition and 

 in a certain sense even to structure. Each plutonic rock has its 

 equivalent in the banded gneisses. 



We pass on now to consider the second point. When large 

 masses of plutonic rock are examined they are often found to vary 

 considerably in composition. They frequently contain patches which 

 differ from the main mass. These have been investigated by Mr. 

 Phillips, so far as the granites are concerned, and have been shown 

 by him to be in general more basic in composition than the rock in 

 which they occur. Again, every one is familiar with the phenomena 

 of contemporaneous or segregation veins. These appear to be in 

 general more acid than the rock in which they occur. Sometimes 

 two or more varieties of plutonic rocks interlace in the most intricate 

 manner. Cases of this kind have been described by Prof. Judd ^ as 

 occurring in the Isle of Eum, where basic and ultra-basic rocks are 

 concerned ; by Messrs. Peach and Home * as occurring in the Shet- 

 land Isles, where granite and diorite are concerned ; and by Herr 

 Lessen^ in the Hartz, where granite (granitite) and gabbro are 

 concerned. The last-mentioned observer states that both granitite 

 and gabbro occur in the same dyke, and that the acid rock either 

 surrounds or penetrates masses of the basic rock in the form of 

 veins— the relations of the two rocks being such as to suggest that 

 the two magmas were intruded simultaneously (or nearly so) rather 

 than that the fluid granite-magma was injected into solid gabbro. 



Diorite and granite may be seen in intimate relation in the ridge 



^ A mass is said to have been strained uniformly when every portion has been 

 strained alike. 



^ Beitrage zur Petrographie, etc. Abh. d. k. preuss. Akad. d. "Wissen. fiir 1869, 

 1873, 1879, and 1884. 



3 On the Tertiary and older Peridotites of the West of Scotland," Q. J". G. S. 

 vol. xli. p. 358. 



* The Old Eed Volcanic Ptocks of Shetland, Trans. Eoyal Soc. Edinburgh, vol. 

 xxxii. part ii. p. 373. 



5 On the Eruptive Eocks of the Hartz, Jahr. d. preuss. Geol. Landesanstalt fiir 

 1882, p. xxi. 



