288 Wisconsin Academy of Sciences, Arts, and Letters. 



by the diminutive term granidite. Aphanite may then be more 

 freely used to include all crypto-crystalline rocks, while ^or^A^/ry 

 ■will embrace combinations of the last with the two former. 



Class IV. Basis of Classification — Physical Characters. 



Trachyte. 



Ehy elite. 



Pumice. 



Scoria. 



PhoDolite. 



Buhrstone. 



Pearlstone, etc. 



Class Y. Basis of Classification — Origin. 



Lava. 

 Trap. 



Meta( .) 



Igno ( .) 



Aguo( .) 



There is a very prevalent, and, for the most part, just prejudice 

 against the use of the name trap, arising from the frequent use of 

 the term as though it conveyed a mineralogical signification, 

 whereas ihe term really has none, and, in its proper application, 

 includes rocks of various mineralogical and chemical constitution. 

 But this abuse is really but an aggravated instance of what is 

 common, indeed, almost universal, under the present system of 

 nomenclature. To merely specify that a rock is granite, may be 

 to use that term as a "cloak of ignorance" in the same sense, 

 though perhaps not to an equal degree, as to rest with ihe asser- 

 tion that a rock is a "trap;" for the term granite embraces a 

 scarcely less wide range of minerals or of ultimate chemical con- 

 stituents, and the wresting of the term from its primitive and 

 proper applicj,tion, is scarcely less violent. If, however, the term 

 trap be stripped of all pretension to mineralogical signification, 

 and confined to the simple designation of rocks formed of matter 

 that issued through iissures, either constituting dikes, or spreading 

 out into sheets, and so incidentally giving rise to step-like topog- 



