SECONDARY ENLARGEMENT OF TOURMALINES. 



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formation of tourmaline after the dynamic sericite, and also after the iron 

 products resulting from the decomposition of some iron-bearing silicate 

 or oxide. In ordinary light these prisms are seen to traverse the tour- 

 maline diagonally and they are certainly included. With a lens the 

 tourmaline can be seen to make the entire thickness of the section. 



Another example of secondary enlargement has the allothegenic core 

 distinctly water-worn and oriented physically with the other elastics in 

 the section and optically with the authogenic addition (see figure 3). 

 The enlargement has nearly complete crystallographic boundaries — a 

 prism doubly terminated ; one end has the planes of a rhombohedron, 



Figure 3. — Thin Section of water-worn tourmaline Pebble. 



Showing secondary growth bounded by nearly complete crystallographic faces. (Drawing from 

 a microphotograph.) 



the other having these and a small basal plane. In this the detrital 

 core is basal, giving a well-marked uniaxial cross of a negative character ; 

 it is of an orange-yellow color and free from interpositions. Owing to 

 its great depth of color, the enveloping tourmaline transmits light only 

 along its edges. Its crystal-faced outline is occasionally interrupted by 

 penetrating grains of quartz, and some grains may be entirely enclosed. 

 A third example shows the dark-colored, secondary mineral penetrat- 

 ing the clastic core nearly to its center along fissures formed prior to this 

 secondary growth. The contrast between the two parts is very distinct, 



