•222 METASOMATIC PROCESSES IN A CASSITERITE VEIN, 



project into cavities. The sample selected for investigation 

 represents a complete transverse section of the vein, showing the 

 change into granite on each side. This sample was cut in halves 

 by a transverse vertical cut. One half is shown in Plate xviii. 



yig.L — Diagrammatic section illustrating how the vein was cut. 



The sections A, B, C, D and E were each one inch in width. Section F 

 was smaller, and only a small slide was obtained from it. 



Text-fig. 1 illustrates how further subdivision was made on the 

 remaining half. The six pieces lettered A, B, C, D, E, and F 

 were formed by cuts, one inch apart. From each of the sections 

 so formed, two slabs were cut. One of these served for making 

 a large section for examination under the microscope; the other, 

 which weighed about 20 grammes or more, was used as a sample 

 for analysis. The slides, which were of large size (about 2x1 

 inches), when examined microscopically, supplied most abundantly 

 all the chief phenomena laid down by Lindgren as evidences of 

 metasomatic replacement. Slide A is from the centre of the 

 vein. Slide G is of the normal granite. The other slides are 

 intermediate and in alphabetical order. A description of the 

 slides is as follows. 



Slide A. — This section is almost entirely composed of quartz. 

 A little secondary haematite is present, filling the fractures in the 

 quartz. Biotite is present in very small amount, and almost 

 invariably shows replacement by a non-ferruginous mica, the com- 

 position of which will be discussed later. The primary quartz 

 occurs both as idiomorphic crystals, and as allotriomorphic grains; 

 and contains abundant cavities, in most of which small liquid 

 inclusions are present. These cavities are very small, and can 

 be seen only under the high powers of the microscope. Most of 

 these cavities are irregular in shape, but many of them are 

 negative crystals of quartz. It is not uncommon to find small 

 crystals included within the negative crystals, but, when this is 

 so, the included crystals are too small to be determined. There 



