Proceedings of the United States National Museum . terials whichwere influenced in their crystallization from solution by the molecularattraction of the same minerals in the adjacent diabase. The finalacid fluid was not stable in contact with these minerals at the closeof the magmatic phase as shown by the evident reaction between thecrystals and the residuum, with replacement of plagioclase by albite,of augite by diopside, and of magnetite by titanite. This rarely ifever reached equilibrium, however, probably because fissuring of themass of the diabase was going forward and the release of the
Proceedings of the United States National Museum . terials whichwere influenced in their crystallization from solution by the molecularattraction of the same minerals in the adjacent diabase. The finalacid fluid was not stable in contact with these minerals at the closeof the magmatic phase as shown by the evident reaction between thecrystals and the residuum, with replacement of plagioclase by albite,of augite by diopside, and of magnetite by titanite. This rarely ifever reached equilibrium, however, probably because fissuring of themass of the diabase was going forward and the release of the pressureof the dissolved gases, which must have been enormous in the laterstages, and upon which the fluid depended for its lowered crystal-lization temperature, caused complete consolidation and release ofthe volatile constituents. These constituents carried with them aload of dissolved solids and continued to react in the same mannerupon whatever material they were in contact with, replacing plagio- ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 41. clase by albite, augite by diopside, and magnetite by titanite. It isconsequently difficult to definitely separate magmatic from hydro-thermal action. The process is clearly a special phase of crystallization differentia-tion, which may be termed pegmatitic differentiation, and presentscertain peculiarities dependingupon the abundant presence ofvolatile constituents in the magmasystem. While there are certainhiatuses in the normal sequencewhich can not now be explained,it is important that the processesare essentially the same as thoseoutlined by Bowen in his discus-sion based upon the investigationof anhydrous systems. It is prob-able that the differentiation ofany natural magma is speeded upand that the reactions are facili-tated by the presence of volatileconstituents. The behavior of the water re-leased after crystaUization of thefinal magmatic product is furtherdiscussed below. MIAROLITIC CAVITIES. Under the term miarolitic cav-i
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