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PROF. S. J. SHAND ON 



[vol. lxxii,. 



no constituent of the black rock can be recognized even with a lens, nor is 

 it possible to say whether the rock is crystalline or vitreous. 



It is in most cases quite impossible to determine whether the veins act as 

 faults. Sometimes a movement of one side of the vein, amounting to not 

 more than 2 inches, can be proved (text-fig. 5, p. 202), but no proof of great 

 displacements was found. Some of the boulders illustrated in PI. XVI have 

 been shifted only an inch or two from the walls which furnished them. 



III. Microscopic Characters of the Pseudotachylyte. 



Sections of most of these rocks must be cut very thin, in order to 

 secure. even moderate transparency. The photomicrograms shown 

 were taken by electric light, with exposures up to 2 minutes, 

 yet the base of the rock appears perfectly black, and only the 

 inclusions exhibit detail. The opacity of the base is due to a 

 multitude of minute black specks (of magnetite), which not only 

 stop a great deal of light, but also make it extremely difficult to 

 determine the nature and properties of the transparent components 

 of the ground-mass. Examination of a large number of slides, all 

 made by myself, has shown that three types of ground-mass are 

 represented, as described below. The inclusions are alike in all 

 cases, and will be described afterwards. 



Type 1. — This is the most opaque type of all, and is found generally in 

 the thinner veins, those less than a couple of inches wide. Clouds of 

 magnetite-grains of irregular shape obscure all detail. Behind these grains 

 one sees a structureless background, which is net perfectly isotropic, but gives 

 dull-grey tints between crossed nicols. The mass is not homogeneous, since 

 different parts do not extinguish at the same time, but boundaries between 

 the different portions cannot be made out ; the appearance is not in the least 

 suggestive of a powder. Some specimens are nearly isotropic, of very dark 

 colour, and show a concentration of the black colouring-matter at many 

 centres. A slight streakiness of the colouring-matter, suggestive of flow- 

 structure, is rarely seen. Fragments of these black rocks, free from visible 

 grains of quartz and felspar, were picked out and placed in bromoform (specific 

 gravity = 2 - 8) ; they all floated. 



Type 2. — In this type a certain amount of crystallization has taken place. 

 The magnetite-grains are in part tiny octahedra, and are accompanied by 

 swarms of grains and scales which are pleochroic from yellowish green to 

 pale grass-green (PI. XIX, fig. 3). These are so minute and so abundant that 

 in the thicker parts of the slides, as also in the specimens of the finest grain, 

 one has merely an impression of greenness over all. Sometimes the prisms 

 form definite collars or mantles around the xenocrysts. The length of the 

 largest prisms does not exceed 0 - 08 millimetre, and the average is much less ; 

 the breadth is from 0"02 mm. downwards. The extinction of the prisms lies 

 between 10° and 20° ; they have high refraction and low birefringence (first- 

 order colours), and they show a perfect cleavage parallel to their length. 

 I am not able to distinguish cleavage in cross-sections, but of all the common 

 rock-minerals the characters just enumerated clearly indicate a hornblende. 

 Tiny scales of brownish-green biotite are also present in less abundance. 

 Through the clouds of magnetite, hornblende, and biotite-grains, one sees 

 again a feebly polarizing background which is either indeterminable, as in 

 type 1, or else passes over towards type 3. 



Type 3. — This type of ground-mass is found in some of the widest veins, 

 but also in some thinner ones, which are not externally different from the- 



