322 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



litlioidal to earthy; also compact and dense. In one locality it is glassy 

 and greenish gray, in part perlitic, passing into dark-gray and black glass, 

 and constituting a pitchstone (683, 684). In nearly every instance it is 

 mottled with what appear to be inclosed fragments having a different 

 character, and which are mostly tuff of the same rock. They are often 

 flattened, giving- the rock a distinct flow structure. The phenocrysts vary 

 in abundance and size in different modifications of the rock. They are 

 sanidine, plagioclase, and biotite. Sanidine is generally perfectly fresh and 

 exhibits a brilliant cleavage surface, while the plagioclase is often decom- 

 posed and is fresh in only a few cases. Biotite is subordinate in amount. 



In places the rock carries many fragments of andesite and of crystalline 

 schist, and passes upward into breccia filled with the latter, and merges into 

 andesitic breccia similar to that already described as underlying- it This 

 relation may be observed at the northern base of Crescent Hill and on the 

 ridge opposite the mouth of Hellroaring Creek. It was the product of 

 eruption of an exceptional modification of magma, rich in sanidine, which 

 occurred early in the period of the extravasation of the hornblende-mica- 

 andesite, when crystalline schists formed the surface of the country through 

 which the eruptions took place. 



This light-colored porphyritic lava consists of an abundant groundmass, 

 which is in nearly all cases megascopically litlioidal and under the micro- 

 scope is highly varied in structure. In this groundmass are numerous 

 phenocrysts of sanidine and lime-soda feldspar, which in numerous cases is 

 decomposed. Ferromagnesian minerals are scarce. The only ones recog- 

 nized megascopical^ are small biotites. In thin sections of the rock 

 pyroxene is seen, in some cases in almost microscopic crystals. A very 

 little green hornblende occurs in a few cases. The sanidine crystals are 

 2 mm. long and smaller, and are usually twinned according to the Carlsbad 

 law. Their cross sections are generally rectangular, but they often have 

 irregular outlines, as though fragments of former well-shaped crystals. The 

 same is also true of plagioclase. The sanidines are very free from inclusions 

 of foreign material ; occasional inclusions of glass, apatite, and zircon occur. 

 The pinacoidal cleavages are often well developed, but some crystals are 

 almost free from cleavage cracks and are easily mistaken for quartz. The 

 lime-soda feldspar is similar to sanidine in size and general form, but exhibits 

 polysynthetic twinning according to albite, pericline, and Carlsbad laws. In 



