382 The American Geologist. meh 
how general they may be. It is, of course, possible that the 
later quartz veins belong to a period entirely subsequent to the 
cooling of the intrusive rocks. 
In thin section, the rock of the tourmaline zones shows 
no features of particular interest. It is simply an aggregate 
of rounded grains of strongly pleochroic tourmaline, with 
quartz and very little feldspar. While no sections have been 
found showing the various steps involved in the passage from 
normal schist to the tourmaline rock, it is evident that the 
process consists essentially of the substitution of tourmaline 
for the biotite of the schist, the latter mineral being abundant 
in the schists but quite lacking in the tourmaline zones. While 
quartz may have been replaced, to a less extent, there is no 
evidence of the fact. 
This substitution of tourmaline for biotite is in perfect 
agreement with the phenomena described by Patton,* but un- 
like the cases described by Lingren,+ where the ferromagnesian 
minerals are unaffected. However the latter instances of 
tourmalinization are in connection with aqueous veins, not 
dikes, and the different conditions have yielded different re- 
sults. 
The granite has no features demanding special considera- 
tion. As already stated, the small dikes contain abundant 
tourmaline which resembles that of the contact rock, but is 
often idiomorphic. The feldspar in these dikes is largely 
microcline. 
In connection with the foregoing facts, interest attaches 
to the localities in the towns of Alexandria and Omar, which 
furnish specimens of pink feldspar and specular iron oxide 
to be seen in many collections. 
About a mile south of the river there is a considerable ridge 
of dark green or gray schist of varying composition, often 
calcareous, and evidently belonging to the sedimentary series. 
This schist is cut through and through by granite in every 
variety of dike, sheet and boss, affording the best example of 
intrusive structures to be seen in this region. Strangely 
enough, contact zones are inconspicuous, but one cannot spend 
an hour on this ridge without being led to think that in it 
Op. cit., p. 24. 
+ W. LINDGREN. Metasomatic Processes in Fissure Veins. Trans. Am. 
Inst. Min. Eng. XXX, pp. 626-644. 
