96 -  GEOGNOSY AND GEOLOGY. 
Section 2. Whitestone (Weisstein). 
This is characterized by compact feldspar, or albite, and the homologous 
species. The whitestone, properly speaking, is a mixture of compact 
feldspar and quartz. The fracture is splintery; the colors greyish, 
yellowish, and greenish-white. Compact feldspar, or albite, is indeed 
characteristic of the whitestone; but where this passes into granite, or 
approximates to it, it assumes a more or less sparry or granular texture. 
Mica occurs as an additional component, and may be recognised by its 
dark color, as also by the lamination which it produces. A variety in 
which grains of quartz and lamine of mica can be distinguished, appears 
similar to several fine-grained modifications of granite, and has received the 
name of granulite. Crystals of feldspar, interspersed in the mass, give it a 
porphyritic appearance. 
Eurite Porphyry. Under this head are to be found some of those red 
and black porphyries, termed quartzose. It is a porphyritic mixture of 
compact feldspar or adinole, with feldspar, albite, or an allied mineral. The 
colors are dirty flesh-color, running into green, and greyish white. 
Claystone , porphyry (porphyre terreux), a porphyritic mixture of a 
substance consisting of compact claystone, of an earthy fracture. It is not 
nearly so hard as eurite porphyry. The presence of silex imparts considerable 
firmness to it. Claystone porphyry is of a dirty flesh, violet, grey, and light 
color. Feldspar is separated either in distinct crystalline particles, or in 
indefinite angular fragments, in which latter case it forms a porphyroid. 
Section 3. Trachytes. 
This section includes rocks which contain vitreous feldspar (sunadin) as 
the principal constituent; the feldspar in most cases, even when recently 
laid bare, appearing as if it had been exposed to the weather. The rocks 
of this section are all of igneous origin. 
Trachyte (trap porphyry, domite, feldspar lava). In this rock sunadin 
predominates, and as it often forms an aggregation of prismatic crystals, 
trachyte is very porous, and rough to the touch. The fresher the feldspar, 
and the greater its proportion, the more lustrous is the rock. A peculiar 
substance, termed andesin (most nearly allied to oligoclase), often replaces 
the feldspar, in that case forming the andesite of many geologists. Mica 
and basaltic hornblende are often contained in it, as also albite, which minerals 
modify the rock in various ways. Oxyde of iron frequently imparts a 
ferruginous tint, otherwise it is of a light color. A crystalline granular 
trachyte is distinguished from trachytic porphyry, porphyroidal trachyte, and 
scoriaceous trachyte. 
The first of the above-mentioned rocks is a crystalline granular aggrega- 
tion of feldspathic crystals, and in Italy, where it occurs very abundantly, is 
called saffomorto, or necrolite. Trachytic porphyry often contains feldspar 
in beautifully perfect crystals of different sizes. They lie in a matrix which 
appears more or less decomposed. Porphyroidal trachyte, instead of crystals 
of feldspar, contains only undefined angular fragments of feldspar. Scoria- 
ceous trachyte derives its name from its appearance. 
Clinkstone (phonolite, porphyry-slate). The petrographical constitution 
526 
