INDIAN CREEK LACCOLITH. 



(31 



chemical composition is given below. The portion of the rock analyzed 

 was the unaltered coarser-grained form (55) 1 occurring in the middle of 

 the laccolith on the north side of Indian Creek. 



Analysis of hornblende-mica- andesiteporphyry. 



[Analyst, J. E. Whitfield.] 



Constituent. 



Per cent. 



SiO 



61.50 



None. 



17.42 



4. 60 



1.09 



Trace. 



1.26 



5.33 



.03 



3.99 



1.29 



.60 



.35 



2.44 



TiO, 



A1 2 3 



Fe,0 3 



FeO 



MnO 



MgO 



CaO 



Li»0 



Na 2 



KO '.. 



P.0 B 



S0 :! 



H.0 



Total 



99. 96 





The main body of the laccolith, where it is about 1,000 feet thick, is 

 a light-gray rock crowded with small crystals of feldspar, mica, and horn- 

 blende, with a subordinate amount of groundmass, whose component grains 

 are not discernible with the naked eye. The phenocrysts are 1 or 2 mm. in 

 diameter and smaller; occasional ones reach 3 mm. The rock is distinctly 

 massive, cracking with irregular joints into angular or somewhat tabular 

 fragments, and exhibiting columnar jointing in only one locality, on the 

 southeast slope of The Dome. Under the microscope the most crystalline 

 portion of the laccolith (57), which proved to be the eastern-central part of 

 the mass on the south side of Indian Creek, is seen to consist of the pheno- 

 crysts already named, cemented together by a holocrystalline aggregation 

 of quartz and feldspar with scattered grains of biotite, hornblende, and 

 magnetite (PL XI, fig. 1). The areas of quartz inclose minute idiomorphic 

 feldspar, in part, if not wholly, lime-soda feldspars, probably oligoclase. 

 The quartz is allotrioinorphic and has a micropoikilitic structure, the grains 



1 Numerals in brackets used in connection with the petrography in this monograph roi'er to the 

 specimen numbers in the Yellowstone Park collection. 



