64 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



has an ill-defined micropoikilitic structure. Among- the phenocrysts are a 

 few irregular individuals of quartz. 



The closest approach to typical andesitic microstructure occurs in the 

 dike cutting shales in the ridge south of Winter Creek, already described 

 (p. 10). The transition from a grounclmass of brown microcryptocrystalline 

 matrix with distinct lath-shaped feldspar microlites and magnetite grains to 

 one that is gray in thin section with larger feldspar laths and a slightly 

 micropoikilitic structure can be observed in one rock section 1J inches 

 long (76). The finest-grain is at the contact with the inclosing rock. The 

 dike is 8 feet wide and the central portion is slightly more crystalline. 

 There is a pronounced fluidal arrangement of the feldspar microlites, more 

 or less parallel to the sides of the dike. The hornblende and biotite are both 

 altered to chlorite, which also fills the centers of the feldspar phenocrysts, 

 leaving a clear marginal zone. Magnetite occurs in phenocrvsts and in 

 minute crystals in the groundmass. Whatever ferromagnesian minerals 

 may have been constituents in the groundmass have been chloritized, and 

 there is no evidence that they were present in any considerable amount. 

 In the rock from the horizontal sheet in this ridge of limestone the ground- 

 mass, which is coarser grained, contains abundant microlites of mica and 

 altered hornblendes, with minute magnetites. These minerals are also 

 abundant as phenocrysts. Apatites and long, thin, doubly terminated 

 crystals of zircon occur. Except for this slightly more ferromagnesian 

 modification of the rock (74), the mineralogical composition of the laccolith 

 is very uniform throughout the whole of its exposure, which covers a dis- 

 tance of 7 miles. 



Segregations occur in places. They consist of comparatively coarse- 

 grained crystallizations of green hornblende, with brownish tones, marked 

 pleochroism, and orthopinacoidal twinning, besides lime-soda feldspar, in 

 part labradorite, with magnetite, some biotite, and a little quartz and grains 

 of calcite; the whole having a hypidiomorphic granular structure. 



MOUNT HOLMES BYSMALITH. 

 DACITE-PORPHYRY. 



The rock constituting this great body, which embraces the mass of five 

 mountain peaks, and is 3 miles long and 2 miles wide, is very uniform in 

 general appearance through the whole extent of the body. It is grayish 

 white, with few small phenocrysts of feldspar and biotite, and has a fine- 



