128 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



THE DIKE ROCKS. 



The dike rocks of Sepulchre Mountain are andesites and dacites, 

 the earliest of which resemble the pyroxene-andesites and hornblende- 

 pyroxene-andesites of the breccias. Mineralogically they range from rocks 

 with phenocrysts of hypersthene, augite, and lime-soda feldspar to those 

 with phenocrysts of quartz, biotite, hornblende, and lime-soda feldspar. 

 This variation is indicated in Table XII, in which 103 sections of these rocks 

 are arranged according to the proportions of the porphyritical minerals. 



Table XII. — Mineralogical variations in the dike rocks of Sepulchre Mountain. 



Mineral 

 group. 



D, 



D, 



D, 



Speci- 

 men 



num- 

 ber. 



421 



422 

 423 

 424 

 425 

 426 

 427 

 428 

 429 

 430 

 431 

 432 

 433 

 436 

 437 

 441 

 442 

 434 

 439 

 440 

 445 

 446 

 447 

 448 

 449 

 451 

 454 

 456 

 457 



Phenocrysts other than feldspar. 



Pyrox- 

 ene. 



Much. 



Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Some. 

 Some. 

 Some. 

 Little. 

 Little. 

 Little. 

 Little. 

 Little. 

 Little. 

 Little. 

 Little. 

 Little. 



Horn- 

 blende. 



Biotite. 



Quartz. 



Trace. 



(?) 

 Little. 

 Little. 

 Little. 

 Little. 

 Some. 

 Some. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. Little. 

 Much. Little. 

 Much. Little. 

 Much. Little. 

 Much. . 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. (?) 



Mineral 

 group. 



D 4 .. 



D 6 



Speci- 

 men 



num- 

 ber. 



D 6 



D, 



li. 



.... 



452 

 455 

 443 

 450 

 453 

 458 

 459 

 460 

 461 

 462 

 463 

 464 

 465 

 466 

 467 

 468 

 469 

 470 

 471 

 472 

 473 

 474 

 475 

 476 

 477 

 478 

 479 

 480 

 481 



Phenocrysts other than feldspar. 



Pyrox 

 ene. 



Trace. 

 Trace. 

 Little. 

 Little. 

 Trace. 

 Little. 



Horn- 

 blende. 



Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 

 Much. 



Biotite. 



Little. 

 Little. 

 Little. 

 Little. 



Little, 

 Little. 

 Some. 

 Some. 

 Some. 

 Some. 

 Some. 

 Much. 



Quartz. 



