HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 575 



base of Mount Washburu, Wyoming, 28975; east side of Snake River, Wyotniug, 

 37273 ; Mararteo, Magdaleua, Michoacan and Rio del Norte, Mexico, 70252, 35780, 

 19536, and 19,547 ; San Juan de los Llanos, Mexico (perlitic), 38130; Guatemala, 

 35587; Alaska, Beach near Nonikaket River, 6384; island of Lipari, 36184 and 

 36143 ; (8 specimens showing common types) ibid., 36284 ; Ascension Island, 36281 ; 

 New Zealand, 36110; Mayor Island, Bay of Plenty, New Zealand. 70345; North- 

 cast Iceland, 72810 ; Mount Gokshi in the Caucasus, 16248; Tokaz, Hungary, 36280; 

 Japan, 11894 ; Granada, Spain (perlitic), 35720. 

 Hyalo-liparite, pitchstone. — North slope of Salient Mountain, Nevada, 35402 ; Esmer- 

 alda district, Nevada, 15567 and 15562; south end of Carson Lake, Nevada, 

 35351 ; Clear Creek, Colorado, 36988 ; Rio Grande, Colorado, 5124 ; Sonora, 

 Mexico, 7058; Grand Canon Yellowstone River (porphyritic), 36830; Spring 

 Valley, Utah (spherulitic), 1025 : Mount Rotaro, Isle of Ischia, 36282 ; Gallatin 

 Valley, Montana, 73116. 



3. The orthoclase or quartz-free porphyries. 



Mineral corn-position.— The essential constituents are the same as 

 those of syenite. They consist therefore of a compact porphyry ground - 

 mass with porphyritic feldspar (orthoclase) and accessory plagioclase, 

 quartz, mica, hornblende, or minerals of the pyroxene group. More 

 rarely occur zircon, apatite, magnetite, etc., as in the syenites. 



Chemical composition. — Being poor in quartz these rocks are a trine 

 more basic than the quartz porphyries which they otherwise resemble. 

 The following is the composition of an orthoclase porphyry from Pre- 

 dazzo as given by Kalkowski :* Silica, 64,45; alumina, 16.31 ; ferrous 

 oxide, 6.49 ; magnesia, 0.30 ; lime, 1.10; soda, 5.00 ; potash, 5.45 ; water, 

 0.85 percent. 



Structure. — Excepting that orthoclase is the porphyritic constituent 

 they are structurally identical with the quartz porphyries, and need 

 not be further described here. 



Colors. — These are the same as the quartz porphyries already de- 

 scribed. 



Classification and nomenclature. — The orthoclase or quartz-free por- 

 phyries bear the same relation to the syenites as do the quartz por- 

 phyries to granite, and the rocks are frequently designated as syenite 

 porphyries. Like the quartz porphyries they occur in intrusive sheets, 

 dikes and lava flows associated with the Paleozoic formations. Owing 

 to the frequent absence of accessory minerals of the ferro-magnesia 

 group the rocks can not in all cases be classified as are the syenites, 

 and distinctive names based upon other features are often applied. 

 The term orthophyr is applied to the normal orthoclase porphyries, 

 and these are subdivided when possible into biotite, hornblende, or 

 augite orthophyr according as either one of these minerals is the pre- 

 dominating accessory. The term rhombporphyry has been used to des- 

 ignate an orthoclase porphyry found in Southern Norway, and in 

 which the porphyritic constituent appears in characteristic rhombic 

 outlines, and which is further distinguished by a complete absence of 

 quartz and rarity of hornblende. (Specimens 1660 and 34831.) The 



*Elemente der lithologie, p. 86. 



