136 



J, J. H. TEALL 01^ THE METAMORPHOSIS OP 



have lost all individual action on polarized light. In the sections 

 which are more or less turbid, the cracks are frequently stained 

 with limonitic decomposition-products. Various modes of twinning 

 may be observed. Simple crystals and binary twins are common ; 

 so also are sections showing very fine multiple twinning and the cross 

 hatching attributed to simultaneous twinning on the albite and 

 pericline types. . , _ 



Optical anomalies due ta strain are frequently recognizable. The 

 extinction-shadows sweep over the sections as the stage is rotated 

 under crossed nicols. The lines separating adjacent twin lamellae 

 are frequently curved ; and sometimes, where the limit of elasticity 

 has been exceeded, a crystal is seen to have been fractured. The 

 twin lamellae often show a great want of persistence in one and the 

 same crystal ; and sometimes they appear to be related to the fractures 

 in such a way as to suggest that they may be, in part, of secondary 

 origin *. 



The pyroxene occurs in irregular grains, imperfect crystals, and 

 plates, which may be either simple or polysynthetic. The lath- 

 shaped felspars sometimes penetrate the pyroxene, and the rock then 

 possesses the ophitic structure of MM. Levy and Touque, 



In the very thin sections examined the pyroxene is either colour- 

 less, or very pale chocolate-brown. The characteristic cleavages, 

 polarization-tints, mode of twinning, and extinction-angles prove very 

 clearly that the mineral is a typical monoclinic pyroxene. 



The bulk-analysis of the rock makes it in the highest degree pro- 

 bable that the mineral is extremely rich in ferrous f oxide. 



The titaniferous magnetic iron-ore occurs in irregular plates, and 

 sometimes in what appear to be skeleton rhombohedra. Apatite is 

 present in the usual colourless hexagonal prisms. Quartz and 

 chloritic minerals may be recognized, both doubtless of secondary 

 origin, and occasionally the pyroxene is seen passing over into green 

 hornblende, the characteristic mineral of the rock now to be described. 



Hornblende-schist. The rock in its typical development is a fine 

 lustrous schist. On a natural face at right angles to the schistosity 

 the foliation is often strongly marked by alternating lenticular bands 

 of dark and light colour, the former being rich in hornblende, the 

 latter in quartz and felspar (see lower part of fig. 1). As a rule, the 

 bands do not show crumpling or puckering ; but in one case this was 

 observed. The individual constituents of the schist are much smaller 

 than those of the dolerite. The average size of the sections of 

 hornblende exposed in a slide cut at right angles to the schistosity, 

 and parallel to the direction in which the long axes of the horn- 

 blende grains are usually arranged, is somewhere about "4 mm. x 

 •15 mm. 



The specific gravities of two specimens of the schist were found 

 to be 3*111 and 3-122. There is therefore a very close agreement 



* Lehmann, 'Die Entstehung der altkrystallinischen Scbiefergesteine,' p. 196, 

 plate c. fig. 1. 



t See "Analysis of Whin Sill pyroxene," Q. J. G. S. 1884, p. 648. 



