88 EOCKS FOBMBD THROUGH IGNEOUS AGENCIES 



6. THE THEEAIiITE-BASAKITE aEOXTP 



This is a small, and so far as now known, comparatively in- 

 significant group of rocks, representatives of wMch are confined 

 to limited and widely separated areas. They are described as 



below: 



(1) THE THBEALITES 



The name theralite, derived from the Greek word Orjpav, to seek 

 eagerly, was given by Professor Eosenbnsch to a class of intru- 

 sive rocks consisting essentially of plagioclase feldspar and 

 nepheline, and which are apparently the plntonic equivalents of 

 the tephrites and basanites. 



The group was founded upon certain rocks occurring in dikes 

 and laccoliths in the Cretaceous sandstones of the Crazy Moun- 

 tains of Montana, and described by Professor J. E, Wolff/ of 

 Harvard University. 



Mineral Composition. — The essential constituents as above 

 noted are nepheline and plagioclase with accessory augite, olivine, 

 sodalite, biotite, magnetite, apatite and secondary hornblende 

 and zeolitic minerals. 



Chemical Composition. — The chemical composition of a sam- 

 ple from near Martinsdale, as given by Professor Wolff, is as 

 follows: Silica, 43.175%; alumina, 15.236%; ferrous oxide, 

 7,607% ; ferric oxide, 2.668% ; lime, 10.633% ; magnesia, 

 5.810% ; potash, 4.070% ; soda, 5.68% ; water, 3.571% ; sulphuric 

 anhydride, 0.94%. 



Structure. — The rocks are holoerystalline granular through- 

 out. 



Colors. — These are dark gray to nearly black. 



The theralit3s, so far as known, have an extremely limited 

 distribution, and in the United States have thus far been re- 

 ported only from Gordon's Butte and Upper Shields Eiver Basin 

 in the Crazy Mountains of Montana. 



(2) THE TEPHRITES AND BASANITES 



Mineral Composition. — The essential constituent of the rocks 

 of this group as given by Eosenbusch are a lime-soda feldspar 

 and nepheline or leucite, either alone or accompanied by augite. 

 Olivine is essential in basanite. Apatite, the iron ores, and 

 rarely zircon occur in both varieties. Common accessories are 



^ Notes on the Petrography of the Crazj Mountains and other localities in 

 Montana, hj J. E. Wolff. Neues Jahrb. fur Min., 1885, I, p. 69; 1890, 1, p. 

 192. 



