30 B. 8. Butler — Pyrogenetic .Epidote. 



These dikes, which occur scattered over several square miles 

 iu this locality, differ somewhat in appearance, chiefly due to 

 difference in weathering, but are very uniform in mineral 

 composition. The freshest specimen obtained was from the 

 dike at the Spread Eagle tunnel. This is a greenish-gray 

 porphyritic rock containing phenocrysts of quartz, plagioclase, 

 altered biotite and epidote. The quartz crystals are not 

 abundant and show marked corrosion. The plagioclase pheno- 

 crysts are very striking, being almost pure white in color and 

 nearly euhedral in form, the larger reaching 8 mm in length. 

 Biotite crystals are rather scattering and show strong chloriti- 

 zation. The epidote occurs in well-formed crystals scattered 

 sparingly through the rock. The largest observed was 12 mtn 

 in length, though most of the crystals do not exceed 5 mm in 

 greatest dimension. They are of sufficient size and abundance 

 to attract the attention at once and were found in every dike 

 of this character examined. 



Under the microscope the quartz phenocrysts show pro- 

 nounced corrosion, having entirely lost their crystal outline. 

 The feldspar crystals in many cases are twinned according to 

 both the albite and pericline laws. Extinction on 010 varies 

 from +7 to +10, with index slightly lower than Canada 

 balsam. These properties correspond to an oligoclase with a 

 composition about Ab 5 An 1# The crystals are clouded with 

 minute dark specks and in some instances there has been con- 

 siderable kaolinization. The biotite has suffered extreme 

 alteration, in some cases to a green pleochroic mica with the 

 separation of iron ore ; in other cases alteration has produced 

 chlorite, epidote and iron ore. In a few instances serpentine 

 has resulted from the alteration. A few crystals of unaltered 

 muscovite or paragonite are present in the specimens. 



The groundmass is composed of unstriated feldspar, with 

 small amounts showing twinning, also of quartz and altered 

 mica. The analyses indicate that the feldspar of the ground- 

 mass is lower in lime than the phenocrysts. Accessory min- 

 erals are epidote, apatite, zircon, and titanite. Many of the 

 epidote individuals evidently once possessed a definite crystal 

 outline, though in most cases there has been enough corro- 

 sion by magma to destroy the sharp crystal faces. In some 

 instances this corrosion has produced embayments in the crys- 

 tals. The contact between the epidote and the groundmass is 

 perfectly definite, there being no lingering out of the epidote 

 into the enclosing groundmass. A few crystals of quartz and 

 apatite are included in the epidote. The included quartz 

 crystals show nearly perfect crystal outline, and have escaped 

 the corrosive action of the magma, which has affected the • 

 epidote and the quartz not thus protected. The evidence 



