292 IRVING. 



for it to be placed among the rhyolites. Described on pages 

 284 and 285. 



4. This rock is the extremely acidic type from loop on 

 the Fremont, Elkhorn and Missouri Valley Railroad, between 

 Texana and Bald Mountain. Except for its intrusive character 

 it is a typical dacite. Described on pages 285 and 286. 



5. Rock described as tonahte, from large dike in Deadwood 

 Gulch. It is intermediate in composition between the granites 

 and diorites, but verges a little toward the syenites from the pres- 

 ence of much orthoclase. Described on pages 286 and 287. 



6. Typical quartz-porphyry, in which the quartz is con- 

 fined to the groundmass, considerable plagioclase is present. 

 Type from Portland Mill exposure. Description and analysis on 

 pages 276 and 277. 



7. Foley Peak type of quartz-porphyry. Contains same 

 constituents as number six but shows a great increase in silica. 

 Quartz phenocrysts are abundant. Described on page 278. 



Between the other rocks, namely the quartz-aegirite por- 

 phyries, the phonolites and the vogesite, the writer has not 

 been able to trace so intimate a connection. There is, how- 

 ever, a much closer relation between the quartz-aegirite rocks 

 and the phonolites than between either of those varieties and 

 the rhyolite-andesite series. 



There is but little doubt that this series of aegirite rocks 

 ■closely parallels the grorudite-tinguaite series of Broegger. It 

 is true that rocks showing the chemical composition of the 

 solrsburgites have not yet been identified, but very few analyses 

 of the more acid trachytoid phonolites have been made, and 

 there is but little question that further investigation will reveal 

 the presence of the intermediate types. 



The difference between these rocks and those described by 

 Broegger are mainly textural. In the grorudites, quartz is con- 

 fined to the groundmass, whereas in the Black Hills, the equiv- 

 alent rocks contain it as frequently in the form of phenocrysts. 

 On the basis of this difference the writer has designated the 

 rock as quartz-aegirite-porphj^ry, thus avoiding the introduc- 

 tion of a new name into the already overburdened science of 

 petrography. 



