Tourmaline Contact Zones.—Smyth. 381 
. 
est variation from the normal granite; and, as above stated, 
they are often a mixture of quartz and tourmaline, or they 
may be nearly pure quartz. Indeed, starting from the nor- 
mal granite, we might expect to find a gradation to a pure 
quartz vein, the latter being filled by hot solutions of silica, 
having their origin in the granite. As a matter of fact, this 
series is pretty well represented, but the quartz as a rule is ac- 
companied by some tourmaline. The latter mineral is, of 
course, indicative of the boric vapors so common in granite in- 
trusions, particularly, as here, characterizing a marginal, or 
contact, facies. 
The small cracks should, according to the above hypothe- 
sis, have relatively the largest tourmaline zones, which, as 
already stated, is the fact. 
The irregular distribution of the tourmaline zones is to 
be accounted for as resulting from varying quantity of boric 
emanations in different fissures, as well as from a varying 
degree of permeability of the schists. Moreover, the irreg- 
ularity is doubtless in some cases, sensibly exaggerated by 
the fact that there is only a small angle between the plane 
of the tourmaline zones, and the surface of outcrop. 
As bearing upon the condition of the magma filling the 
fissures, one further fact may be added. In the tourmaline- 
granite filling cracks a few inches wide, the tourmaline is in 
well-defined prisms, evidently the first of the essential min- 
erals to crystallize. Practically all of these prisms lie ap- 
proximately at right angles to the course of the fissure. Had 
the tourmaline crystallized before.the magma was forced into 
the fissures, the prisms would, beyond question, tend to lie 
parallel to the sides. It is thus evident that the fissures were 
filled by an entirely fluid material and that all of the minerals 
of the granite, with the possible exception of some accessories, 
have crystallized in their present position. 
While the foregoing explanation would account for the 
phenomena as practically contemporaneous, there can be no 
doubt that the filling of minor fissures by deposition from 
heated solutions would continue long after the actual period 
of intrusion. In this way pegmatitic dikes and veins would 
form which would cut the earlier filled fissures. Such rela- 
tions are actually seen in the field, but it is impossible to say 
