SECONDARY ENLARGEMENT OF TOURMALINE. 



175 



the garnet has been sufficient to supply the staurolite and leave a resi- 

 due, which appears as magnetite. 



Tourmaline and its secondary Enlargement. — This mineral is specially 

 abundant in the lowest schist bed, occurs usually in black prisms, and 

 in size varies from microscopic dimensions to several centimeters. In 

 the pegmatite veins it is often found in knots as large as one's fist, in- 

 closed in quartz. It is less abundant in both the other horizons, but 

 appears not infrequently in crystals, which are just discernible under 

 the lens. Under the microscope the only noteworthy characters of this 

 tourmaline are the absorption and a marked zonal structure without per- 



Figure 4. — Portion of a Crown of Staurolite and Magnetite encircling a garnet Individual. 



In ordinary light. X 33. 



ceptible gradations in color. There are nearly always a core and an en- 

 circling rim. The core has usually pleochroism in blue to plum tones, 

 whereas the rim shows brown tones like biotite. Sometimes the outline of 

 the core is parallel to the bounding planes (figure 1,0). In the graphitic 

 mica schist of the second railroad cut west of Ore Hill (number 3534) 

 undeniable enlargements of tourmaline are found. One of these is rep- 

 resented in figure 1, B. The core is oval, and is surrounded by a halo 

 of cloudy opaque material (graphite). It shows the brown tones like 

 biotite. About this rounded and probably detrital core a stoutly colum- 



XXVI— Bull. Gkol. Soc. Am., Vol 4, 1802. 



