308 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



west of this cliff on the south side of the valley are greatly obscured by soil 

 and drift. The scattered exposures show the breccia highly indurated, and 

 a more crystalline form of intrusive rock. The coarsest-grained variety 

 is a medium-grained diorite (1554), which was not found in place. Several 

 other forms of diorite are found in the morainal accumulations (1552, 1553); 

 also a porphyritic variety (1555) with hornblende phenocrysts. At the north 

 base of Grizzly Peak fine-grained diorite-porphyry, with small phenocrysts 

 of hornblende and some biotite, forms a large body extending for about a 

 mile in an east-west direction (1550, 1551). It is blue-gray and compact. 



A broad dike of hornblende-andesite, 75 feet wide, can be traced for a 

 long distance up the northeast spur of Grizzly Peak. It trends north of 

 east. Along the crest of the ridge from Grizzly Peak to Top Notch at 

 least 16 dikes have been observed, trending' at various angles toward the 

 southwest, south, and southeast. One is a 40-foot dike of hornblende-mica- 

 andesite (1556) trending northeast. It is dark gray, with abundant pheno- 

 crysts of feldspar, altered hornblende, and fresh biotite. Five others are 

 of hornblende-andesite. Dikes also cut the upper end of Signal Ridge. 

 On the saddle west of Top Notch Peak a dike of hornblende-andesite, 

 trending northwest and southeast, is intersected by a 30-foot dike of horn- 

 blende-mica-andesite (1557), which trends N. 10° E. The rock is dense 

 and dark colored, with numerous small phenocrysts of feldspar, decomposed 

 hornblende, and fresh biotite. On the ridge southeast of the summit of Top 

 Notch Peak there are two dikes of hornblende-andesite trending;- S. 20° E., 

 which appear to extend across the head of Middle Creek in a southeasterly 

 direction; east of them, on the spur, between the forks of this creek, are 

 several other dikes with the same general trend toward the southeast. The 

 summit of Top Notch Peak is traversed by dikes in several directions. A 

 25-foot dike of hornblende-mica-andesite (1558) trends S. 20° E. The rock 

 is light gray, and is filled with phenocrysts of feldspar, biotite, and horn- 

 blende, and a few of quartz. Another of this kind of andesite trends with 

 the ridge of the summit. A dike of hornblende-andesite east of the summit 

 trends N. 15° E., and two others occur northeast of the summit. 



All of these dike rocks are noncrystalline with the exception of two, 

 which may be glassy and crowded with microlites or may be holocrystalline. 

 When studied microscopically they are found to be, one, pyroxene-andesite 

 with paramorphs after hornblende (1499); five, hornblende-pyroxene- 



