32 Descriptwe Arrangement of Volcanic Rocks. 



uate so imperceptibly into one another, that two genera alone 

 can hardly be reckoned sufficient. 



A very numerous tribe of rocks is to be met with, in which 

 the pro[)ortions of felspar and augite are such, that it is im- 

 possible to tell which predominates, while, in their general 

 characters, the rocks are so unlike the extremes of either ge- 

 nus, that it is scarcely allowable to rank them together. It 

 appears from these reasons expedient to institute an inter- 

 mediate class of rocks, for the reception of those members 

 which cannot, without difficulty, be referred to either of the 

 two extremes. The name which I conceive most appropri- 

 ate to them, as having been applied to such rocks already 

 by Werner and other mineralogists, and as susceptible of lit- 

 tle misconception, is greystone, (graustein,) their color being 

 universally of some tint of grey, generally lead-grey, green- 

 ish, iron, purplish, or slate-grey, with the exception only of 

 their vitrified varieties, some of which have assumed a black 

 color, which, however, passes away under the blowpipe, and 

 is succeeded by the usual grey tint.* 



The genera of the volcanic, or trap family of rocks, will 

 then consist of — 



I. Trachyte. 

 II. Greystone. 

 III. Basalt. 



It may, perhaps, be objected to the basis of this arrange- 

 ment, that these subjects frequently appear homogeneous, 

 and hence their constituent minerals are undiscoverable. 

 This is, however, far from the case. There are very few 

 rocks of this family, indeed, in which a good lens, or at fur- 

 thest, a microscope, will not discover a granitoidal mixture 

 of the constituent minerals in a crystalhne form. The meth- 

 od of mechanical analysis, first proposed by M. Cordier, will 

 determine this with accuracy and certainty. But, for ordi- 

 nary purposes, examination with a lens will be sufficient, and 

 even the color may be generally depended on as an accurate 

 criterion, unless the rock is passing to a resinous or vitreous 



* Greystone corresponds in part to the class of volcanic rocks called te- 

 phrine, by M. de la Metherie. It comprehends also the majority of clink- 

 stones. 



