ORIGIN OF THE PEGMATITES OF MAINE 307 



principle outlined above is eventually shown to be operative to an 

 important degree in magmas. It is perhaps unnecessary to add that 

 other causes besides low molecular weight may be effective in re- 

 ducing viscosity in magmas. 



Field and laboratory evidence hearing upon viscosity and gas 

 content. — The field and laboratory data on the pegmatites of Maine 

 which bear upon the viscosity or gaseous content of the pegmatite 

 magmas may be set forth as follows. Since the pegmatite magmas 

 crystallized at some distance below the surface, the gases which 

 they contained must either have made their escape through the wall- 

 rocks or else have remained in cavities or occluded within the solid 

 pegmatite mass. The escape of such materials through the wall- 

 rocks should presumably leave some record in contact metamorphic 

 effects. Their retention within the rock should presumably be 

 recorded in an especial abundance of miarolitic cavities and fluid or 

 gaseous inclusions. The field studies of the writer in Maine and 

 other parts of New England show that the granites are almost wholly 

 devoid of miarolitic cavities of any kind. An isolated cavity of small 

 size is occasionally met with but its walls are usually more or less peg- 

 matitic in texture. In the great bulk of the pegmatites of Maine, par- 

 ticularly the finer-grained ones, such cavities are also exceedingly 

 rare. In the coarser pegmatites, however, they are a characteristic 

 feature, though usually, as far as can be judged, constituting consider- 

 ably less than i per cent of the total volume of the pegmatite. Within 

 the very limited gem-bearing zones of certain pegmatites miarolitic 

 cavities may form a considerably larger percentage of the total volume. 

 Such cavities have been attributed by various writers to shrinkage of 

 the pegmatite mass in crystallization. This may in fact play some part 

 in their formation but that they are not entirely the result of shrinkage 

 but on the contrary were filled or partly filled with some material which 

 has since disappeared, is shown by the presence of perfectly developed 

 crystals of quartz, tourmaline, and other minerals projecting inward 

 from the walls of the cavities. Some filling must have been present 

 from which such crystals derived the materials for their growth. It 

 is probable therefore that immediately after the crystallization of the 

 main body of pegmatite the miarolitic cavities were completely filled 

 with a gaseous solution which may later have liquefied and has since 



