Origin of Pegmatite. — Croshij and Fuller. 157 
line of demarcation. Tliis contact zone of the pegmatite 
masses is, perhaps, the most usual phase of the banded struc- 
ture to which the advocates of the aqueous tlieory attach so 
much importance. Brogger says,* "The zonal structure, when 
any such is present, which, however, is generally not the case, 
usually makes itself evident only in a liner-grained condition 
of the vein boundaries, and sometimes (especially in the case 
of granitic pegmatite veins) in a zone with graphic structure 
next the fine-grained eugranitic marginal zone, upon which 
there frequently follows (especially in acid granitic pegma- 
tite veins) in the middle of the vein a tremendous size of 
grain, here often with special enrichment in rarer minerals 
and also (particularly in acid veins) not seldom with open or 
distinctly drusy cavities filled with peculiar mineral deposits." 
All this accords with our observations; but, besides this 
pseudo-banding, there is a true 'vein-banding closely akin to 
that of undoubted aqueous formations. This, however, is of 
rather rare occurrence; and, according to our observations, 
mostly confined to the narrow, linear, crack-filling veins, such 
as abound in the Concord granite. We are unable to agree 
with Br()gger that this banding is alwa3^s distinct from that 
of true veins, '-inasmuch as the outer zones pass continuously 
into the inner." We will only cite a single instance; but this 
appears to be typical. In the Museum of the Boston Society 
of Natural History is a complete cross-section specimen from 
a well-defined linear vein of pegmatite in Chesterfield, Mass. 
The width of the vein is six to seven inches: and it is made 
up of two sharply defined and symmetrical bands or laj^ers of 
albite (clevelandite), with the tabular crystals set edgewise 
to the walls, and a median band of smoky quartz: while 
springing directly from either wall and penetrating both the 
albite and quartz are numerous slender prisms of green and 
red tourmaline. The order of crystallization is clearly marked ; 
and it is very obvious that in this instance the successive pe- 
riods or generations did not overlap. The minerals were de- 
posited strictl}^ in succession, although for that matter a 
certain amount of contemporaneous deposition is comnjon 
enough in true veins. 
The disposition of the tourmalines noted above is highl}'' 
characteristic; and may be observed in many massive or un- 
*Canadian Record of Science, 6, G6. 
