506 
ELEME^^TS OF GEOLOGY. 
Clinkstone or Fhonolite. —^Among the rocks of the tra- 
chytic family, or those in which the feldspars are rich in 
silica, that termed Clinkstone or Phonolite is conspicuous by 
its fissile structure, and its tendency to lamination, which is 
such as sometimes to render it useful as roofing-slate. It 
rings when struck with the hammer, whence its name ; is 
compact, and usually of a grayish blue or brownish color; is 
variable in composition, but almost entirely composed of 
feldspar. When it contains disseminated crystals of feld¬ 
spar, it is called Clinkstone porphyry. 
Volcanic Rocks distinguished by special Forms of Structure. 
—Many volcanic rocks are commonly spoken of under names 
denoting structure alone, which must not be taken to imply 
that they are distinct rocks, L 6., that they differ from one 
another either in mineral or chemical composition. Thus 
the terms Trachytic porphyry, Trachytic tuff, etc., merely re¬ 
fer to the same rock under different conditions of mechan¬ 
ical aggregation or crystalline development which would be 
more correctly expressed by the use of the adjective, as por- 
phyritic trachyte, etc., but as these terms are so commonly 
employed it is considered advisable to direct the student’s 
attention to them. 
Porphyry is one of this class, and very characteristic of the 
volcanic formations. When distinct crystals of one or more 
minerals are scattered through an 
earthy or compact base, the rock 
is termed a porphyry (see Fig. 
586). Thus trachyte is usually 
porphyritic; for in it, as in many 
modern lavas, there are crystals 
of feldspar; but in some porphyries 
the crystals are of augite, olivine, 
or other minerals. If the base be 
greenstone, basalt, or pitchstone, 
the rock may be denominated 
greenstone - porphyry, pitchstone- 
Porphyry. White crystals of feld- POlThy^, and SO forth. The old 
spar in a dark base of hornblende classical type of this form of roCk 
and feldspar. porphyry of Egypt, or 
the well-known Rosso antico.” It consists, according to 
Delesse, of a red feldspathic base in which are disseminated 
rose-colored crystals of the feldspar called oligoclase, with 
some plates of blackish hornblende and grains of oxide of 
iron (iron-glance). Red quartziferons porphyry is a much 
more siliceous rock, containing about 70 or 80 per cent, of 
silex, while that of Egypt has only 62 per cent. 
Fiff. 586. 
