BY W. G. WOOLNOUGH. 513 



tion varies from sky-blue tints up to a maximum which is fairly 

 high for chlorite. 



A considerable quantity of epidote is formed at the same time. 

 In colour this is yellowish-green, and is easily recognised b}^ its 

 high refractive index, and strong double refraction, in grains 

 scattered throughout the chlorite. 



The onl}^ inclusion of importance is magnetite in grains and 

 crystals. 



Biotite is less abundant than hornblende, but is nevertheless 

 an important essential constituent. It occurs in thoroughly 

 allotriomorphic sections up to about 1-5 x 0-75 mm. in area. 



Cleavage, refractive index, and double refraction are quite 

 normal. As stated above, the mica plates are often bent owing 

 to the strain which affects the rock as a whole. The pleochroism 

 is as usual very strong; vibrations parallel to the cleavage are 

 almost completely absorbed, while those vibrating at right angles 

 give a strong yellow colour. 



The mica shows the effects of decomposition to a much greater 

 extent than the hornblende does. The earliest stages are marked 

 by the occurrence of odd patches of chlorite in parallel position 

 with regard to the original mineral. All stages between this, 

 and a complete replacement of the mica by a mixture of chlorite, 

 epidote, and opacite, can be traced. 



The chlorite is similar to that described above under the 

 decomposition products of hornblende. In the final stage it is 

 usually impossible to say to which of the two species the original 

 mineral belonged. As a rule the epidote derived from the biotite 

 is rather lighter in colour than that from the hornblende, but 

 it is otherw^ise very similar. 



Included in the biotite are grains and crystals of magnetite. 

 Though intimately associated, the hornblende and biotite are 

 not intergrown to any extent. Where any trace of such an 

 arrangement occurs, the mica appears to be the older of the two. 

 In addition to the small grains of magnetite included in the 

 ferromagnesian minerals, this mineral is fairly abundant in more 

 or less idiomorphic sections throughout the rock. 



