J. J. H. Teall — Origin of Banded Gneisses. 485 



rocks is of two kinds. It may consist (1) of a parallel arrangement 

 of certain constituents {e.g. mica plates or porpliyritic felspars), or 

 (2) of an alternation of bands of varying chemical and mineralogical 

 composition. It is agreed on all hands that a parallel structure of 

 the first kind may be due either to the deformation of a mass of 

 half-consolidated plutonic rock at the time of intrusion, in which 

 case it is strictly analogous to the flow structure in many volcanic 

 rocks, or to deformation produced by earth-stresses operating on the 

 mass after consolidation. As regards the origin of the second kind 

 of parallel structure there is no such general agreement, and many 

 writers regai'd it as due to some process akin to sedimentary deposi- 

 tion. The object of the present paper is to show that just as the 

 first kind of parallel structure may be and in certain cases actually 

 is due to the plastic deformation of what may be termed homo- 

 geneous ' plutonic masses, so the second kind of parallel structure 

 (banded structure) may be and in certain cases actually is due to the 

 deformation of heterogeneous plutonic masses. 



In the first place let us inquire what kind of deformation is 

 necessary in order to produce parallel banding in a heterogeneous 

 mass. A moment's consideration will show that if any heterogeneous 

 mass of fairly uniform dimensions in the different directions be 

 deformed into a flat sheet or narrow ribbon, then the individual 

 portions will be similarly deformed and the mass as a whole will 

 show a banded structure. This was clearly realized by Mr. Poulett- 

 Scrope in 1824, who not only explained in this way the parallel 

 banding of the Ponza liparites, but also suggested that the similar 

 appearances in gneiss and schist might be due to the same cause.^ A 

 very simple experiment will serve to illustrate the important fact 

 just referred to. Take pieces of differently coloured clays (say 

 ordinary potter's clay and clay which has been intimately mixed 

 with the blue used by laundresses) : bring them together in any way 

 you please so as to make one lump and deform the lump into a thin 

 sheet or narrow band by squeezing or rolling. Then fold the sheet 

 or ribbon on itself two or three times and, if you like, repeat the 

 process of rolling it out. In this way the most perfect parallel 

 banding may be produced and by varying the conditions in ways 

 which readily suggest themselves (as for example by using clays of 

 different degrees of plasticity and inserting fresh pieces after the 

 deformation has progressed to a certain extent) it is easy to produce 

 many of the peculiar structures seen in banded gneisses. 



It is important to note that the result will differ according as the 

 mass is deformed into a flat sheet or narrow strip. In the former 

 case the strain-ellipsoid will be an ellipsoid of revolution with its 

 short axis in the direction in which the stress was applied, in the 

 latter case the strain-ellipsoid will possess three unequal axes, 

 the least axis lying approximately as in the former case, and the 

 greatest axis lying at right angles to this and in the direction of 



^ Not homogeneous in the strict sense of the term, hut homogeneous in the sense 



that the different mineralogical constituents are uniformly mixed throughout the mass. 



2 Geology of the Ponza Isles, Trans. Geol. Soc. London, 2nd ser. vol. ii. p. 228. 



