of ee Sa) a F : eit tet ome S 
PS el “ya : . 
a = r , 
Parr Il. § vi] CRYSTALLINE ROCKS—MASSIVE. 135 
and in this respect differs from the superficial volcanic rocks, such as 
lavas. which have flowed out above ground from volcanic orifices. 
Quartz-Porphyry (Quartz-felsite)..—Under this title are included 
several varieties of rock which agree in consisting fundamentally of 
a very fine grained felsitic ground-mass, composed mainly of ortho- 
clase and quartz. Where these minerals are crystallized in con- 
spicuous forms the rock is a quartz-porphyry (felsite-porphyry, eurite) ; 
where the whole mass is more homogeneous and flinty in texture 
it is a felsite or felstone. 
Quartz-porphyry is composed of a compact ground-mass through 
which are dispersed crystals or crystalline blebs of quartz and 
erystals of orthoclase, sometimes of a triclinic felspar, mica or horn- 
blende. Though to the eye in fresh specimens the ground-mass often 
appears homogeneous and almost flinty in texture, it generally presents 
under the microscope the microfelsitic structure already described 
(p. 104). Sometimes the base is found to be distinctly glassy, while 
in other cases it appears partly glassy and partly microfelsitic. 
Occasionally it assumes a more crystalline character, even sometimes 
recalling the structure of a fine grained granite. Beautiful examples 
of spherulitic structure are occasionally to be observed where minute 
spherical concretions occur with an internal fibrous radiating structure. 
Fluxion-structure is well developed among some of the quartz-por- 
phyries associated with the metamorphic rocks of the north-east of 
Scotland. 
The quartz occurs in imperfect occasionally corroded crystals or 
blebs, but sometimes in pertect doubly-terminated pyramids, varying 
in size from minute forms only discernible with the microscope, up 
to crystals as large as a bean. It abounds with liquid inclusions. 
The orthoclase takes the form of more or less complete crystals, not 
seldom twinned; the contour which its cross sections present to the 
eye, depending upon the angle at which the individual crystals are 
bisected. It is chiefly the dispersed orthoclase which gives the dis- 
tinctively porphyritic aspect to the rock. Triclinic felspar (believed 
to be usually oligoclase) also takes a place, distinguishable when 
fresh, by its fine lineation, but apt to become dull and kaolinized by 
weathering. Mica and hornblende are among the most common of 
the minerals which accompany the two essential constituents, while 
apatite, magnetite, and pyrite are not infrequent accessories. 
The flesh-red quartz-porphyry of Dobritz, near Meissen, in 
Saxony, was found by Rentzsch to have the following chemical 
composition :—Silica, 76°92; alumina, 12°89; potash, 4°27; soda, 
0°68; lime, 0°68; magnesia, 0°98 ; oxide of iron, 1:15; water, 1:97 ; 
total, 99°54,—specific gravity, 2°49. 
The colours of quartz-porphyry depend chiefly upon those of the 
felspar,—tiesh-red, reddish-brown, purple, yellow, bluish or slate- 
grey, and even white, being in different places characteristic. The 
presence of much mica or hornblende gives dark grey, brown, or 
1 Zirkel, Microscop. Petrog. p. 71. See particularly Rosenbusch, Mik. Phys. ii. p. 50. 
