BIGHORN PASS SHEET. 69 



tered, extends through the groundmass for some distance as single crystals, 

 inclosing many quartz crystals, in a poikilitic manner. 



The same kind of contact zone exists east of Mount Holmes and west 

 of Trilobite Point. The plane of contact is nearly vertical, and from the 

 aphanitic marginal zone numerous offshoots penetrate the adjacent rocks. 

 The dikes are white, aphanitic, and exhibit banding and flow structure. 

 Specimens from the contact were studied and found to be very fine grained, 

 with micropoikilitic structure, the quartz individuals being about 0.15 mm. 

 in diameter, and having a skeleton-like form, the outline of each quartz 

 being nearly idiomorphic, but not continuous, as shown in PL XI, fig. 4. 

 The feldspar forms minute clouded grains and crystals. There are micro- 

 scopic flakes of muscovite scattered through the rock, and some calcite. In 

 the bysmalith rock immediately in contact with the andesite-porphyrv of 

 the laccolith the microscopic skeleton quartzes are scattered in a micro- 

 cryptocrystalline groundmass. Calcite is abundant in irregular grains. 

 The aphanitic rock penetrates the limestone in sheets that sometimes break 

 into thin crumpled layers. This modification is microgranular, very fine 

 grained, and not poikilitic, and consists of quartz and feldspar in allotrio- 

 morphic grains. The size of the grains varies slightly in alternate layers, 

 producing the lamination. Minute flakes of muscovite are scattered through 

 the rock and intersect one another at all angles. They are more abundant 

 in some layers than in others. 



Similar offshoots of microgranular rock occur on the north side of 

 Panther Creek and near the ridge west of the head of Grallatin River. In 

 these bodies, however, biotite is more abundant, and the micropoikilitic 

 structure passes into micrographic structure as the feldspar inclosures 

 assume a more uniform orientation (96, 97, 98). 



BIGHORN PASS SHEET. 

 KERSANTITE. 



The small obscure body of dark-colored porphyry-like rock which is 

 exposed in the vicinity of Bighorn Pass is characterized by phenocrysts of 

 hornblende and mica, and rarely those of feldspar. In places the horn- 

 blendes are quite large ; in other parts of the mass there are no phenocrysts 

 (124). On the pass the sheet is from 50 to 75 feet thick. On the north 

 side of Three River Peak there is a nearly horizontal intrusive sheet, 10 feet 



