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iVas a rule green in colour and either uralitic or actinolitic. Specks and 

 granules of opaque iron-ore are frequently associated with the uralitic 

 aggregates which are less markedly pleochroic than the actinolitic horn- 

 blende. Occasionally a pale brown uralite may be observed. The 

 original felspar, when present, gives more or less turbid lath-shaped 

 sections ; very frequently, however, the felspar is represented by water- 

 clear aggregates composed of large irregular individuals, or else showing 

 mosaic structure (see Fig. 2, Plate XXL). That these aggregates are 

 secondary is proved by the fact that they are often crowded with needles 

 of secondary hornblende. The only ether essential mineral in these rocks 

 is titaniferous iron-ore, often more or less changed to leucoxene. A pale 

 brown mica, distinct from that of the hornblende-biotite-dolerites, is very 

 common as an accessory constituent. Tourmaline, garnet and axinite are 

 less common. Apatite is usually present and secondary quartz is by no 

 means rare. 



A rock from the White Tor near Tavistock furnishes a good example 

 of a tolerably advanced stage of metamorphism. It consists of horn- 

 blende, felspar, titaniferous iron-ore, pale brown mica (contact mica), and 

 a little tourmaline. The hornblende is mostly uralitic or actinolitic. 

 The uralite is pale in colour, sometimes showing a decided brown tint. 

 The typical actinolite is strongly pleochroic ; a, pale green ; /3, green ; 

 7, bluish-green. Dots and specks of magnetite are common in the feebly 

 pleochroic but absent from the strongly pleochroic varieties of hornblende ; 

 a fact which, as LOSSEN remarks, points to the conclusion that the 

 pleochroism increases with the amount of iron (Fe 2 3 ), in the hornblende 

 molecule. The uralitic aggregates are penetrated by pseudomorphs 

 after lath-shaped felspars ; a point of great importance, as it proves that 

 the original rock was an ophitic dolerite. The spaces between the patches 

 of uralitic hornblende are now principally occupied by a colourless matrix, 

 sometimes granular and sometimes water-clear, in which detached needles 

 and groups of actinolite crystals are extremely abundant. This matrix is 

 an aggregate of irregular grains, mostly untwinned, of secondary felspar. 

 The general relation of the hornblende and felspar in those portions 

 of the slide which do not show a marked ophitic texture is similar to that 

 represented in Fig. 1, Plate XX. The titaniferous iron- ore occurs in ex- 

 tremely ragged plates and is exactly similar to that of the normal ophitic 

 diabases. It preserves its form and character better than any other 

 mineral. In studying metamorphosed dolerites (diabases) the present 

 writer has found the titaniferous iron- ore and its characteristic alteration 

 products, leucoxene and granular sphene, of the greatest use. They will 

 frequently give a clue to the origin of even a foliated rock when almost 

 every other trace of the original character has disappeared. 



The above rock from the White Tor is a typical epidiorite. The pale 

 brown mica occurs in aggregates and the individual scales are somewhat 

 larger than those of the Tolcarn rock represented in Fig. 2, Plate XVII. 

 Sections at right angles to the basal plane are colourless when viewed with 

 rays vibrating at right angles to the cleavage cracks and a pale brown when 



