814 THE JOURNAL OF GEOLOGY. 
It is hoped that an opportunity for working this up more in 
detail will present itself in the near future. In the meantime I 
shall rely on the following brief description, and more especially 
upon the accompanying photographs (PI. II., Figs. 1 and 2), to 
PLATE II. 
Fic. 1.— Micropoikilitic quartz-porphyry, showing the peculiar structure of the 
groundmass in ordinary light. Magnified 18 diameters. 
give a correct idea at least of the appearance of it under the 
microscope. 
The structure of the groundmass can be seen even in ordinary 
light; it is brought out better when the field is partly shaded, so 
as to bring out the varying relief of the minerals (PI. II., Fig. 1); 
and best of all between crossed nicols (Pl. Il., Fig. 2). The 
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