I50 A. F. BUDDINGTON 



lined with terminated quartz crystals and well-bounded crystals of 

 microperthite projecting into the cavity. In the miarolitic cavities 

 of the granite porphyry masses of Ernest Sound, hexagonal biotite 

 crystals are common in addition to the quartz and feldspar. About 

 12 miles farther to the northwest, on Zarembo Island, are the 

 small masses of white rhyolite porphyry intrusive into slates. The 

 rhyolite porphyry is exceptionally miarolitic, and the volume of the 

 druses, as measured on the polished surface of a hand specimen, is 

 about 4^ per cent. The druses vary from the fraction of a centi- 

 meter to a centimeter in diameter. These druses are lined with 

 terminated quartz crystals projecting into the cavity, and are associ- 

 ated or flecked with many small plates of crystalline hematite 

 (specularite). The druses are further partially or completely 

 filled with a microcrystalline granular aggregate of kaoHnite 

 (Av. w = i.564=t:.oo3), and the central portion is usually filled with 

 calcite. In some specimens kaolinite is absent and calcite alone 

 fills the central portion. 



If we consider the texture of the different facies of the rock to 

 indicate the physical conditions during consolidation, an interesting 

 correlation may be made between these conditions and the 

 miarolitic structure and mineral association of the druses. The 

 medium-grained granite with only a faint trace of porphyritic 

 texture is compact, and presumably cooled so slowly that opportu- 

 nity was given for the mineralizers in solution to escape gradually. 

 The granite porphyry may be considered to have suffered an abrupt 

 change in its rate of cooling, probably accompanied by increased 

 viscosity, so that either the released gases or residual fluid were in 

 part temporarily trapped, and deposited the pneumatolytic minerals 

 in the druses. The mineral association of the druses — quartz, 

 microperthite, and biotite — is characteristic of high temperature 

 and pressure. The rhyolite porphyry represents a still more abrupt 

 change in rate of cooling and a lower drop in temperature. The- 

 abrupt crystallization attendant upon this may have resulted in an 

 increased vapor pressure, resulting in larger and more prominent 

 druses than in the granite porphyry. The mineral association of the 

 druses — quartz and specularite — is one such as is found in the early 

 stages of the formation of zeolites in surface flows. 



