OLDEK EOCKS OF BEITTAKTT. 



309 



runs roughly parallel with the axes of the loops, but where they 

 make with it the largest angle it is least definite, and here trans- 

 verse cracks, apparently giving relief from strain, traverse the 

 lighter bands. 



I have examined slices cut from four varieties of this series, two 

 of them representing the above -de scribed varieties taken at the 

 " wriggle," the third, a slab rather iutermediate between the two 

 varieties, and the fourth a coarser kind which exhibits very distinct 

 banding, zones of mica alternating with felspathic layers, from a 

 quarter to half an inch or more wide. 



The principal minerals in the first rock are felspar, quartz, mica, 

 iron oxide, and an earthy-looking substance which, for brevity, I 

 will call ferrite. The first two are by far the most abundant, and 

 the felspar rather exceeds the quartz. The felspar appears to be 

 orthoclase ; at any rate, the lamellar twinning, usually characteristic 

 of plagioclase, is absent. The felspar occurs in rather elongated 

 grains of irregular outline (PI. XVII. fig. 1); many of these show 

 a cleavage roughly perpendicular to the longer axis of the grain. 

 The larger grains are much interrupted by inclusions of quartz, 

 usually very clear. The form of these varies much : sometimes they 

 are rounded or oval spots, occasionally one or more of the boundaries 

 is rectilinear, and they sometimes tend to be arranged in ill-defined 

 streams in the direction of the longer axes of the felspar ; thus at 

 first sight parts of the slide might be taken for a kind of mosaic of 

 rather small independent grains of quartz and felspar, but on apply- 

 ing the nicols the uniformity of tint indicates the not unfrequent 

 presence of a single crystal of felspar interrupted by the quartz 

 grains. There are also some elongated streaks of quartz formed of 

 a few long interlocking grains which show strain-shadows. The 

 quartz, too, tends to parallelism in the direction of its grains, and, 

 speaking of the slide as a whole, the minerals exhibit a rather 

 general uniformity of tint, as if this arrangement held throughout 

 in the individual constituents. The mica, of which there is very 

 little, is sometimes brown, sometimes olive-grey; irregular wavy 

 cracks, marked out by ferrite, traverse the slide roughly in the 

 direction already mentioned. The minerals in the second slide are 

 the same, except that there is less quartz and a good deal of an 

 olive-grey mica, in flakes often about *03 inch long. There are also 

 some smaller flakes of brown mica. As in the last slide, the felspar 

 grains are interrupted by secondary quartz. Cracks, indicated by 

 ferrite, traverse the slide, giving parts of it almost the aspect of a 

 true fragmental rock. The third slide contains similar minerals, the 

 mica occurring in fairly thick and well-defined bands. In the fourth 

 the arrangement is similar, but the quantity of mica in different parts 

 of the slide is variable, and in parts there is marked evidence of 

 crushing. 



The mica described above is of a pale olive-grey colour, sometimes 

 almost colourless, often streaked with darker lines and with plates 

 (haematite ?) between the cleavage-planes. It is slightly dichroic, 

 and, notwithstanding its paleness, I am disposed to regard it as an 



