256 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



of feldspar, quartz, and. mica. This definition is totally independent of 

 any varying form of genesis or method of appearance. From the in- 

 formation that we have gained through the study of erupted rocks it 

 seems eminently probable that varying results may be derived from the 

 same original magma by different methods of cooling. To jiresent an 

 extreme case, we may assume that the same magma can, in one instance, 

 be productive of the formation of high silicates mainly, while if cooling 

 under different circumstances the silicates may be lower and free silica 

 from the complement. If we speak, therefore, of eruptive granite, we 

 mean to denote a rock that to-day has the mineralogical constitution of 

 granite, totally irrespective as to what its previous condition may have 

 been. What effects are in reality produced by the different methods of 

 cooling we are unable to say. Certain experiments point to one view, 

 others to another. It becomes a matter of great difficulty to arrive at 

 definite conclusions from investigations that must necessarily be carried 

 on on a small scale, when we desire to apply the knowledge thereby 

 gained to masses of enormous j)roportions. 



Proceeding to the remaining eruptives of Colorado we find that the 

 adopted scale is more completely filled than by the preceding groups. 

 An important member of the series — propylite — has not been recognized 

 in Colorado. Eichthofen has found it well developed farther west, and 

 in Transylvania it plays a very prominent part. The propylite of Offen- 

 banya was originally named " timazite" by Bielz. It is the rock contain- 

 ing the telluride ores of Transylvania. Cotta* states that he regards 

 Its age as younger than Eocene. His evidence for this assertion is de- 

 rived from the fact that the same ore-bearing veins traverse the tima- 

 zite and Eocene sandstones. In Nevada and adjacent regions the rock 

 has been found, and is regarded by Eichthofent as older than andesite. 

 With reference to this latter term there is some confusion, unless it be 

 limited, as has most recently been done. The name was originally ap- 

 plied to all the volcanic rocks of the Cordilliera system, and not until a 

 short time ago has its application been clearly defined. Andesites occur 

 in the volcanic regions near Colorado, occupying a definite geognostic 

 position. They are younger than propylite, older than the trachytes. 



Trachyte is a name that has been very indiscriminately aijplied to many 

 light-colored volcanic eruptives. Within its scope have been placed the 

 volcauics of Transylvania, of Germany, France, and, to a large ex- 

 tent, of our western continent. Varieties have been based upon the pre- 

 domination of one or the other inclosed mineral. Thus oligoclase, sani- 

 dine, and amphibole trachytes have been distinguished. Although the 

 presence or absence of one or the other of these minerals is undoubtedly 

 of systematic value within the circumscribed areas, no rule can be es- 

 tablished that would answer" in all cases. 



Ilauy first gave this name to the volcanic rocks of the Auvergne. 

 Trapp-porphyry was adopted for the same rocks. L. von Buch gave that 

 group of trachytes occurring near Puy de Dome the name domite. Beu- 

 dant-, who made the first extensive examinations of the volcanic regions 

 of Transylvania and Hungary, separated the eruptives into four groups :| 



Trachyte, 



Porphyre tracJiytiqiie, 



Per lite, and 



Porphyre trachytique molaire. 



* Treatise on Ore Deposits, Prime. New York, 1870, p. 277. 



tMem. Gal. Ac. Sci., 1868, vol. i, partii. 



tBeudant, Voyage Mioeralogique at G6ologique eaHongrio, 1822, Tom. iii, p. 310. 



