ENDLicH.] COMPARISON OF EEUPTIVES. 257 



From his descriptions the first agrees with our present trachyte, and 

 the second with rhyolite. Perlite is of subordinate importance, and the 

 fourth probably indicates a variety of rhyolite. Timazite he has evi- 

 dently included in his trachyte. 



In Germany the volcanic rocks of the Drachenfels and other regions 

 allied to it have passed under the names of trachyte and trachyte-tufl's. 

 A variety of trachyte occurring in Silesia has been named teschenite by 

 Hohenegger.* It is composed of amphibolite, augite, nephelite, and 

 triclinic anorthite. Its age is regarded as being Eocene. 



Throughout the Western country the application of the name trachyte 

 is becoming more and more restricted. For the case that it may be 

 impossible or inexpedient to separate the most closely allied rock-spe- 

 cies, I have introduced the more comprehensive term of " trachorheite." 



Trachyte occurs in large masses not only in Colorado, but also in other 

 States and Territories of the West. Numerous varieties might be dis- 

 tinguished by special names, but there would be no end to the treasures 

 of nomenclature were such separations attempted at the present time. 

 A characteristic of trachyte is the constitutional change it may undergo 

 within a very limited space. King describes an instance of this kind 

 from the vicinity of the Comstock Lode.t He says, " It is of the sanidin 



variety," " partly a pinkish breccia, very loose in texture, 



and partly of a grayish, shaly sandstone mass, which is unlike any of 



the trachyte forms observed in the Great Basin," " but, as 



it approaches the main trachyte, becomes more and more crystalline, and 

 pavsses imperceptibly into the ordinary variety, without any apparent 

 junction line. Certain specimens cannot be separated from metamor- 

 phic sandstones." Such occurrences are not rare within the trachyte 

 areas, and their explanation may most properly be sought in the diifer- 

 ent methods of cooling. It remains to be observed whether the char- 

 acter of the rock is such as has, in previous pages, been designated as 

 symmorphic. 



It has been an impossibility for me to find any definite analogy for 

 what I distinguish as trachyte No. 4. Its similarity to timazite, both 

 as a rock and in its associations, has been pointed out, but its geolog- 

 ical position precludes the assumption of any identity. 



Important as a portion of the entire series are the inclosures of the 

 trachyte. No applicable rule governing their distribution can be given. 

 In Colorado and elsewhere obsidian, pumice, and pitch-scone are charac- 

 teristic of the trachyte group. Occurring in nodules or bands, they 

 impart a peculiar habitat to the rocks containing them. Varying in 

 composition from their immediate surroundings, they vary also in struct- 

 ure. Allusion has above been made to the concentric structure of a 

 dike of obsidian observed in 1873.| An analogous case occurs in Bo- 

 hemia, near Teplitz,§ where porphyry shows concentric arrangement, 

 and weathers so as to simulate a half-opened rose. Whitney has ob- 

 served similar conditions near Old Carson Pass.|| There basaltic lava 

 has been poured oat over metamorphic granite. Through the action of 

 heat, and probably pressure, the latter '' has a concentric structure de- 

 veloped in it." 



Porphyritic trachyte is a term which has first been applied by Mr. Mar- 

 vine to a definite type of eruptives in Colorado. In speaking of Park 



*F. Eoemer, Geologie von Obertchlesien. Breslau, 1870, p. 363. 



tGeoloK. Exp]. 40th Parallel, vol. iii, 1870, p. 36. 



i Eep. U. S. Geol. Saw., 1873, p. 345. 



$ Gotta., Geologische Biltler. Leipzig, 1871, p. 187. 



U Geol. Surv. Cal., 1865, vol. i, p. 447. 



17 a 



