16 PROCEEDINGS OE THE NATIONAL MUSEUM vol.66 



In the former case it may be presumed that there was no means of 

 escape for the contained water and it was retained until final con- 

 solidation, the expulsion of the water being the result of crystalliza- 

 tion. In the case of hypabyssal intrusions, forced into magma cham- 

 bers at moderate depth, on the other hand it may be conceived that 

 the surrounding rock was to some extent permeable, permitting the 

 escape of some of the vapors and corresponding reduction of the 

 vapor pressure of the magma. Such action might be expected to 

 give general contact action by the magma on its walls proportional 

 to the porosity or permeability of the confining rock at any given 

 point. If this rock were limestone the extent to which it was aifected 

 would be dependent on its permeability, a property not directly con- 

 nected with its chemical or mineralogical composition. 



Such loss of volatile constituents of the igneous mass, by permea- 

 tion of the enclosing walls, results in a decreased vapor pressure in 

 the mass of fused material and a lessening of the content of dis- 

 solved gases. It would thus act to constantly raise the point of con- 

 solidation of the magma and, taking place concurrently with loss of 

 heat by diffusion into the surrounding rocks, would inevitably hasten 

 the final consolidation. Since the presence of phenocrysts, carried 

 already crystallized in the magmas filling many such bodies, pre- 

 cludes the idea that they were greatly superheated when intruded, 

 the combined influences would tend to crystallize them rather quickly 

 with little opportunity for further differentiation. At the crystal- 

 lization of the mass as a whole, however, the remaining volatile con- 

 stituents, including the remainder of the water must be expelled, 

 either through the consolidated rock as a mass or through fissures 

 which might be developed from the act of crystallization or by some 

 outside agency. If the final elimination of water took place uni- 

 formly without fissures it might be expected to continue to move 

 as the earlier emanations had moved, controlled by the permeability 

 of the surrounding rocks. In most cases, however, fractures seem 

 to have developed at the critical moment, giving localized channels 

 for the escape of the materials. These late emanations doubtless 

 were laden with materials in solution and were capable, in their 

 earlier and hotter stages, of producing lime-silicate mases like those 

 resulting from the earlier emanations. Whether the materials car- 

 ried in solution at a given place outside the magma were the original 

 constituents of the solutions at the moment of crystallization or 

 whether they are the result of reactions and substitution in the trav- 

 ersed rock, where they have produced alterations so generally as to 

 be difficultly demonstrable, can not in all cases be determined. Seri- 

 citization of the feldspar seems a common effect of such late solu- 

 tions and this can be detected, but minor substitutions might take 



