kaxsomk J METAMORPHISM OF THE COUNTRY ROCK. 121 
altered form of the foregoing latite. The original porphyritic struc- 
ture is perfectly preserved, but the rock is bleached to a very light- 
*ray tint, and the feldspars are completely kaolinized. Small crystals 
)f pyrite are abundant, occurring both in the altered phenocrysts and 
in the groundmass. This is evidently the country rock which imme- 
liately incloses the ore, although it is not known how far this pro- 
nounced alteration extended on either side of the ore-bearing fissure 
sone. Under the microscope the rock appears as a fine-granular 
tggregate, consisting chiefly of quartz, kaolin, and diaspore, with 
considerable disseminated pyrite. The porphyritic structure is par- 
ially lost between crossed nicols, although the abundance of diaspore 
n some of the altered phenocrysts differentiates them from the 
groundmass. The diaspore occurs in irregular grains up to about 0.5 
nillimeter in diameter. The relatively high refractive index, strong 
louble refraction with consequent brilliant polarization colors, and 
he conspicuous cleavage render the mineral very noticeable in thin 
section. It is biaxial, with dispersion red less than blue. 
In order to con firm the microscopical determination, a portion of 
he rock was powdered and treated with hydrofluoric acid by Dr. 
lillebrand. The resulting residue proved, on microscopical exami- 
lation, to consist of grains of the supposed diaspore, with a few 
^articles of pyrite and a little amorphous material. A rough qualit- 
ative analysis showed that the residue consisted of about 84.4 per 
sent of alumina and 15.2 per cent of water, thus placing the identifi- 
•ation of the mineral as diaspore beyond question. 
A chemical analysis of this altered rock is given in Column II on 
•age 122. The two analyses of fresh and altered rock, as they stand 
ude by side, represent a comparison of unit weights, and therefore 
ire not, in all probability, direct ly comparable for the purpose of ascer- 
aining the actual change that has taken place in a unit volume of the 
iriginal rock. It may, however, be tentatively assumed that the 
ilumina has remained constant. An analysis of the altered rock 
•ecalculated on this basis gives the result shown in Column Ha. A 
comparison between Column Ila and Column I shows that, under the 
assumption made, the altered rock has suffered a total loss of substance 
)f over 14 per cent. A little silica, 3.15 per cent of water, and some 
iulphur have been introduced, while magnesia and carbon dioxide 
lave been wholly, and lime-soda potash almost entirely, removed. 
)f the original iron present in the rock, a little more than one-half 
•emains combined with sulphur to form pyrite. It will be noted that 
n the recalculated analysis the phosphoric and titanic acids are very 
learly the same as in the fresh rock, which lendb some support to the 
issumption previously made that the alumina has remained practi- 
^alty constant in the process of metamorphism. It seems at first sight 
irobable that the loss of substance in the altered rock may be accom. 
lanied by an increase of porosity without actual shrinkage of the rock 
is a whole. If this be so, the specific gravities of the unaltered and 
