258 iRViNi.;. 



basis of tlicsc ililt'orciu'os. Vhc \\\\<.cvosco\'ic luis shown th.it, 

 althoui^h ot diltcront .ippc.u.uu'o in the tioKl, tlio luinor.ilo'MiMl 

 composition is pr.ictic.illy tho s.inu\ .is also is {\\c (.luMuir.il i'imu 

 position. llonoo. with ono hio.iil .uul lalht-i arhitrai\' ili\ ision 

 separating;' tho phonolit(.\'^ proju-r, aiul those ot tin^-naitic toxtiiro 

 from those ot' trachxtoiil taeics. the niinoraloiMcal anil eluMuioal 

 characters ol" the dilt'erent \arieties will he ileseiihed ti'>L;tiluM". 

 allusion heini; niaile to tlu' separate oeeunenees imi1\' wheie 

 some unusual feature seems to merit special mention. 



Before attem[-)tini; a ileseription of these rocks, it wouKl he 

 well to hrictly iletuie the terms used for the ditferiMit species. 



The [ihonolitic rocks ha\e been L;iouped unika' the tollow in^;" 

 licails : 



1. Tiuguaite. 



2. Phonolite proper. 



3. Tracliytoid phonolite. 



Tinguaites. 



I'onccinin^:' the term tiuguaite one cannot read tlu^ liti-iature 

 of the phonolites \vithout feeling' that consiilerablc ciMifusioii 

 has cxistcii. Rosenbusch orii^inalK' pioposed the teini tor those 

 rocks that formed the dike t\pe oi the nephcline s\enite series. 



The type rock was tlcnsc, i^reenish and non poriilu'ritic, and 

 consisted o( an interlockiuL;' ma/e o( a\L;irine nectllcs with san- 

 idine and some nephcline. IMionolite was the extrusiw^ rcprc- 

 sentatixe and consisted of a porplniitic rock w ith phenocr\'sls 

 c^f sanidine and accessor\- a^i^irine in a L^roundmass oi sanidiiu^ 

 and automorphic neiiheline. 



Broegger, howcxer, has usctl tiuguaite, not onl\- as a tcxtural 

 term, but to desi^^nate the basic members of his groruilite-tin- 

 guaite series. This usage is, therefore, parti)' chemical. There 

 is, then, a two-fold division of the rocks, one o\\ geological and 

 the other on chemical characters. 



In Broegger's use of the term the woi'ii represents a fairly 

 definite group of rocks, and in that sense the more basic of the 

 Black Hills phoiu>lites are tinguaites. As regards the other cm- 

 geological usage, the facts obserx'cd in the hills (\o not alti>- 

 gether justify its employment. 



