PLATE 10 



Direct Colour Photomicrograph. 



Screen-plate duplicating process (Paget). 



SECTION OF QUARTZ FELSITE x 10, under polarised light. 



Objective Zeiss 50 mm. Planar F. 4.5. 



Ocular None. 



Condenser ... Back lens of Watson's Universal, with polariser 



and Selenite fitted underneath. 

 Illuminant ... Liliput 4 amp. arc. 

 Aux. Condenser Double convex lens, giving parallel rays. 



Screen Wratten & Wainwright's Kj 



Plate Paget colour plate. 



Exposure 1 sec. 



The specimen shows phenocrysts of quartz, more or less corroded 

 at their edges, set in a microcrystalline ground mass of quartz and 

 felspar. The darker parts point to the presence of ferro-magnesian 

 minerals. 



A. chip from the rock is ground down and polished until extremely 

 thin to produce the transparent preparation. 



Photo. W. B. R. ( To follow page 96. 



