260 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



rhyolite and basalt. There are three distinct periods of basaltic eruptions 

 and an indefinite number of minor outbursts of the same rock. The study 

 of the whole region shows the frequent recurrence of certain lavas. 



Among the intrusive rocks the mineralogical variations differ according 

 to the phase of crystallization of the rock. In the finer-grained dike rocks 

 they range from olivine-augite-labradorite-basalts, through those with little 

 or no olivine, to hornblende-pyroxene-andesites and to still more siliceous 

 mica-hornblende-pyroxene-andesite-porphyries, and in another direction to 

 more basic rocks, which range from olivine-augite-mica rocks, through 

 auarite-mica rocks, to auoite-hornblende-mica rocks with orthoclase and 

 plagioclase. 



There is also a mineral variation which accompanies the degree or 

 phase of crystallization of the rock. It is illustrated by the transition from 

 basalt to gabbro. The character of the variation will be shown by a com- 

 parison of the mineral composition of rocks whose chemical composition has 

 been determined to be similar. 



Analyses of rocks from the Crandall volcano. 



Constituent. 



1 



1 



2 



3 



4 



- 5 



6 



7 



8 



SiO, 



1 

 ... 51.81 



52.09 



.39 



17.84 



4.27 



4.56 



.14 



5.33 



8.03 



3.39 



1.98 



.27 







52.11 



.53 



16.58 



3.66 



4.99 



.23 



6.87 



6.43 



3.25 



3.20 



.63 



53.56 



.68 



16.07 



3.21 



5.29 



.11 

 7.23 

 8.77 

 3. 06 

 1.94 



.18 



53.71 



.74 



18.00 



3.99 



4.05 



.24 



5.19 



6.88 



3.50 



3 10 



.38 



53.89 



.49 



18.81 



4.92 



2.81 



.17 



3.29 



5.12 



3.65 



2.98 



.52 



55.93 



.81 



18.32 



2.39 



4.91 



.14 

 3.97 

 6.17 

 4.29 

 2.62 



.56 

 



56.21 



.88 



18.24 



3.26 



3.69 



. 17 

 3.38 

 5. 91 

 4.15 

 3.02 



.64 







Al-O-s 



... 15. 24 



F&1O3 



... 3.66 



FeO 



MuO 



MgO 



CaO 



Na.O 



4.86 



j .08 



8.89 



9.06 



... 2.83 



P„0 6 



.18 



MO 





CI 













H„0 



.67 



1.77 



1.99 



.19 



.55 



2.99 



.22 



.78 

 100. 33 



Total 





100. 13 



100. 06 



100. 47 



100. 29 



100. 33 



99.94 



100. 33 



1. Gabbro-porphyry ; core on Hurricane Mesa, Craudall Basin (1388). 



2. Basalt; flow, north side of Timber Creek, Crandall Basin (1252). 



3. Basalt; dike, ridge south of Hurricane Mesa, Crandall Basin (1325). 



4. Basalt-porphyry; core, Hurricane Mesa, Craudall Basin (1383). 



5. Gabbro (with mica); core, Hurricane Mesa, Crandall Basin (1430). 



6. Basalt-andesite glass, breccia; ridge south of Indian Peak, Crandall Basin vl241). 



7. Orthoclase-gabbro-diorite (rich in mica); core, Hurricane Mesa, Crandall Basin (1399). 



8. Orthoclase-gabbro-diorite (rich in mica); core. Hurricane Mesa, Crandall Basin (1396). 



