242 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



the crystal. Cross sections are square, with large square inclusions of 

 groundmass. Ilmenite occurs in many of the rocks in rod-like crystals, and 

 a small amount of serpentine is present in most of the groundmasses. 



Olivine is decomposed to serpentine in most instances, and originally 

 formed small as well as large crystals. In one of the fresher rocks the 

 groundmass contains many small olivines, which are probably the mineral 

 from which the serpentine in the groundmass of the more altered rocks was 

 derived. As a whole, the basaltic dikes are not so fresh as the breccias 

 surrounding them, although they are the younger rocks. Those with large 

 tabular feldspars acquire a very characteristic appearance through the 

 whitening of these crystals, which are strongly contrasted with the dark 

 dense groundmass. The large dark-colored augites are also distinctly 

 noticeable. 



Gabbro- and diorite-porphyries and andesites. The l"Ocks which WOldd be placed 



under this division in consequence of a microscopical examination probably 

 belong quite as closely to the basalts. With one exception they all occur 

 in close proximity to the granular core, and some of them are included in 

 it. The one exception is a narrow dike of pyroxene-andesite, the ground- 

 mass of which is filled with serpentine, indicating the former presence of a 

 magnesian mineral, possibly olivine. 



The rocks of this division exhibit all of the modifications of mesra- 

 scopical habit shown by those just described, and resemble them closely in 

 hand specimens. They are, however, more crystalline and present micro- 

 structures both distinctive and characteristic, which are related to differences 

 in mineral composition. A few of the andesitic dikes which cut the summit 

 of the plateau west of the core are very fine grained and are considerably 

 altered, and contain chlorite and epidote. 



The absence of olivine from most of the more crystalline forms of 

 these rocks appears to be due to the causes which influenced the crystalli- 

 zation of the rocks and not to their chemical composition, for the hand 

 specimens in some cases show what seem to be decomposed crystals of 

 olivine, which in thin section are found to be paramorphs after this mineral. 



This group of rocks includes the intrusive sheets on top of Hurricane 

 Mesa and certain dikes. In the immediate neighborhood of the core and 

 within the zone of indurated breccia the massive sheet of intrusive rock 

 appears dense and aphanitic (1359, 1361, 1369), and carries abundant tabu- 



