BY W. N. BENSON. 537 



recognisable by their high refractive index and low double 

 refraction. Tourmaline also occurs, but is not abundant; scarcely 

 a dozen crystals are present in the t3'pe-slide, though other 

 members of the group are richer in this mineral. The crystals 

 are oriented quite without reference to the cleavage plane. In 

 some members of this group a little orthoclase appears to be 

 present. The usual accessory minerals are rutile in fairly short 

 crystals, zircon, and apatite in very small prisms. 



Mr. H. J. Meldrum, B.Sc, has analysed the slate in the quarry 

 on Portion 42, a very typical example of this group of slates. 

 He obtained the following figures (A). 



(A) 



SiO,, 68-67 



AI2O3 16-99 



Fe,03 l-ll 



FeO 1-87 



MgO 2-34 



CaO 2-33 



Na.^O 1-07 



K.^O 2-88 



TiO^... present n.d. 



H.,0 1-92 



99-18 99-39* 



Analysis B, showing the composition of a biotite slate from 

 Cross River, Minnesota, U.S.A., is added for comparison. It 

 will be seen that the analyses are very similar, the Newbridge 

 slate being richer in silica but poorer in alkalies than the other. 



This is the normal slate of the district, being simply the 

 original sediment consolidated. All the other types of slate, 

 except the Augen and Felspathic, may be regarded as metamor- 

 phosed Blue Slate. 



The varieties of this first group are Green Slate and Oxidised 

 Slate. The former is partly oxidised Blue Slate, and is perhaps 

 the most common slate of the whole area. As will be seen on 

 the map (fig.4), it occurs between Newbridge Station and the 



* Rosenbusch, Elemente der Gesteinlehre, p. 51 3. 



