PETEOGEAPHY— EHYOLITE, TRACHYTE, AND PHONOLITE. 483 



thread-like appearance of the general mass of the rock determines it to be 

 serpentine, which a blowpipe test confirmed. It has probably been formed 

 by an alteration of the slates, which theory an exact observation of its 

 occurrence would perhaps support. 



SECTION III, 

 EHYOLITE, TEAOHYTE, AND PHONOLITE. 



The recent eruptive rocks, formerly known under the name of the 

 trachyte group, have of late been divided into several classes, depending 

 upon their structure and the presence or absence of the minerals, quartz, 

 and oligoclase, or other triclinic feldspars. 



The quartz-bearing trachytes are called rhyolite and quartz-trachyte, 

 while those free from quartz are subdivided into sanidin-trachyte and 

 sanidin-oligoclase-trachyte, accordingly as the sanidin occurs in them alone 

 or accompanied by oligoclase. In close relationship and analogy to these 

 rocks are andesite, consisting generally of oligoclase and hornblende or 

 augite, and phonolite, which latter has sanidin for its chief constituent, with 

 nephelite, nosite. and various decomposable zeolites. While sanidin-oligo- 

 clase-trachyte, by the addition of quartz, falls into the class of the acid 

 trachytes or rhyolite, so on the other hand, by the gradual diminution of 

 the sanidin, it becomes andesite. In this rock the oligoclase occurs in com- 

 bination with hornblende as well as augite, and andesite is accordingly 

 divided into hornblende-andesite and augite-andesite. As with trachyte, 

 quartz is sometimes an ingredient, and causes a subdivision of both kinds 

 of andesite into quartz-bearing hornblende or augite-andesite, the former 

 being called dacite, and those free from quartz. 



The glassy forms of trachyte vary great!)' in appearances and struc- 

 ture, and are known as obsidian, trachytic-pitchstone, pearlite, spherulite, 

 &c. They may be regarded as masses of the same composition as the 

 varieties of trachyte already mentioned, but solidified under circumstances 

 preventing a crystallization of their constituent minerals. As ma} T be 

 inferred, the} 7 vary in every respect similarly to the crystalline member of 



