262 REPOET UNITED STATES GEOLOGICAL SDEVEY. 



Table sJiowing the classification of volcanic eruptivesfor the Bocky Mount- 

 ains and adjacent regions. 



Note. — Those occurring in Colorado are printed in small caps, and the synonyms for other countries 

 in italics. The arrangement begins with the oldest group. 



volcanic eruptives. 

 Basic : Acidic : 



Propylite. 

 Syn. : Timasite, HuDgary, 



Gi'iinstein-trachyt of German geologists. 



Andesite. 



Syn. : Andesite of the Cordilliera system. 

 TEAcnvTE : 

 Trachyte No. 1. 



Syn.: TrachyUotvffa,iraoJiyt-tuffolGeT- 

 many. 

 Trachyte No. 2, and 

 Trachyte No. 3. 



Syn.: Sanidine and oUgoclase trachyte. 

 Domite of the Auvergne. Trachyt of 

 Germany. Trapp-porjihyr by Buch. 

 Trachyte by Beudant, of Hungary. In- 

 closures of these trachytes are : Per- 

 lite,ol)sidian,pitch-stone,a,ndpori}hyritio 

 pitch-stone. 

 Trachyte No. 4. 

 Ehyolite. 



Syn.: Nevaditeoi Western regions. Bhyo- 



Ziie and (?adte of Transylvania. Quartz- 



, porpliyr of older German geologists. 



Forphyre trachytique of Hungary, by 



Beudant. 



Porphyritic trachyte. 



' In the present acceptation this has no 



synonym. Liparite expresses what for- 

 merly was termed porphyritic trachyte. 

 Diabase. 



Syn.: Trapp of Germany and Eastern 

 States. 

 Dolerite. 



Syn.: Trap}). Varieties are termed wepfte- 

 lindolerite of Germany, and sanidine-dole- 

 rite of New Mexico. 

 Leucitophyrite. 



Syn.: iewa/opftj/r of Germany and France; 

 leucite-rock of Green Eiver Basin. 

 Basalt. 



Syn. : Trapp of Germany and Eastern 

 States. Varieties of basalt are meZa- 

 phyr, &c. luclosures : OUvinyte, &c. 



OEIGII!^ OF ERUPTED EOCKS. 



At tbe close of the description of the erupted rocks of Colorado, a 

 brief discussion of the origin of such material may here find its place. 

 It is natural that the study of the products of plutonic and volcanic 

 activity will create in the mind of every student some conception as 

 to the forces to which such products owe their existence and present 

 position. It is equally natural that every individual mind may grasp 

 more particularly one point observed that might have escap'ed the at- 

 tention of others. Thus the result evidently vs^ill be that each writer 

 on the subject will show a personal coloring illustrative of the predomi- 

 nant direction towards which bis views incline. It is not m^^ intention 

 to present, at the close of this paper, an exhaustive review of the vari- 



