

92 



circumstances sometimes appear identical when examined with light 

 vibrating parallel to the ,< axis. 



The hornblende of the rocks under consideration may occur in the 

 form of large plates without any definite outline, exactly like the ophitic 

 plates of augite, or as crystals showing definite form. The colourless variety 

 may occur as fine needles (piUi<> of I>ECKK), or as good sized plates which have 

 a silky appearance in consequence of their finely fibrous structrue. 



The brown variety is sometimes found intimately intergrown with 

 augite, and this relation will be described in detail when the Inchcolm rock 

 is referred to. The two minerals appear to be related to each other in such 

 a way that the vertical axis and plane of symmetry are common to both. 

 Biotite. This mineral occurs as an accessory constituent of certain picrites. 

 In the normal condition it is deeply coloured, and sections at right angles 

 to the basal plane show a most intense pleochroism. In certain varieties of 

 picrite it appears to have undergone a peculiar bleaching process, in virtue 

 of which it has become almost colourless. (1) The characters of this nearly 

 colourless biotite as it occurs in the curious rock from Achavarasdale ^loor 

 (Scyelite) have been described by Professor Jumx It is the most conspicuous 

 mineral when this rock is viewed in hand specimens, although by no means 

 the most abundant. The fracture of the rock is largely determined by it, 

 and large flat glistening surfaces, often measuring one square inch in area, 

 are produced whenever the rock is broken. These surfaces have a pale 

 bronzy colour and a submetallic lustre. The hardness of the mineral is 

 about three of Moils' scale. The specific gravity is '2-8. The mineral is 

 almost infusible before the blow-pipe, and is scarcely acted upon by boiling 

 hvdrochloric or sulphuric acids. Its approximate chemical composition 

 is : 



Si0 2 ... 38-o 



ALO, ... 13-0 



FeO (Fe a 3 ) ... 4-5 



CaO ... ")( 



MgO ... 24-0 



H 2 O ... 6-5 



K , and Na 2 (?) 3-0 (by diff.j 



Thin flakes examined in convergent light show that the mineral is 

 practically uniaxial, resembling in this respect a pale silvery biotite from 

 Easton, Massachusetts, described by BLAKE. (2) When thin slices of the 

 rock are examined under the microscope, sections of the mineral at right 

 angles to the basal plane show a faint pleochroism of the type characteristic 

 of biotite, and opaque inclusions, probably of a tabular form, are seen in 

 certain parts of the mineral lying parallel with the planes of easy cleavage 

 (see Fig. 2, Plate V.) These inclusions strongly suggest that the original 

 mineral has been bleached by secondary actions (" schillerisatioii") involving 



(1) JUDP. Q.J.G.S., Vol. XLL, p. 402. 

 See also WILLIAMS. A.J.S., 1886, p. 30. 



(2) A.J.S., Vol. XII., (1851), p. 6. 



