
128 | GEOGNOSY. 
rock is broken the fracture passes between them, and the new surface — 
obtained presents innumerable dull rounded grains. 
Besides occurring in alternation with schists, quartzite is also — 
[ Boox abs & 
met with locally as an altered form of sandstone, which when tra- i 
versed by igneous dykes is indurated for a distance of a few inches 
or feet from the intrusive mass. These local productions of quartzite 
show the characteristic lustrous fracture, and have not yet been 
distinguished by the microscope from the quartz-rock of wide 
metamorphic regions. There is yet another condition under which ~ 
this rock or one of analogous structure may be seen. Highly 
silicated bands, having lustrous aspect, fine grain, and great hard- 
ness, occur among the unaltered shales and other strata of the Car- 
boniferous system. In such cases, the supposition of any general 
metamorphism being inadmissible, we may infer either that these 
quartzose bands have been indurated, for example, by the passage 
through them of thermal silicated water, or that they are an original 
formation. 
Schistose Quartzite (Quartz-schist).—The gradation from quartz- 
rock into the various schists can be traced in almost any region of 
_ metamorphic rocks. It is perfectly analogous to the passage of 
sandstone into shales and other sedimentary formations. The 
Highlands of Scotland consist in large measure of rocks which are 
not properly either mica-schist or ordinary quartz-rock. Consisting 
of granular quartz, with abundant parallel laminze of mica, and 
capable of being split into thick or thin flagstones, they may be 
called quartz-schists. They were evidently at first sandstones, with 
interleaved seams of fine mud. ‘The sand has been converted into 
quartzite, and the argillaceous layers have passed into various 
micaceous minerals. Endless varieties in the relative proportions 
of these ingredients may be observed. eee 
Itacolumite.—A schistose quartzite, in which the quartz- 
granules are separated by fine scales of mica, talc, chlorite, and 
sericite. Occasionally these pliable scales are so arranged as to give 
a certain flexibility to the stone (flexible sandstone). This rock 
occurs in the south-eastern states of North America, also in Brazil, 
as the matrix in which diamonds are found. 
Halleflinta (Helleflinta)—An exceedingly compact felsitic grey, 
yellowish, greenish, brownish, or black rock, composed of an intimate 
mixture of microscopic particles of felspar and quartz, with fine 
scales of mica and chlorite. It breaks with a splintery or con- 
choidal fracture, presents under the microscope a finely-crystalline — 
structure, and is only fusible in fine splinters before the blow-pipe. 
Though externally presenting a resemblance to felsite, one of the 
massive rocks, it occurs in beds so intimately associated with the 
gneisses of Norway, that it has probably been produced by the same 
series of changes that gave rise to the crystalline schists, 
Porphyroid.— A name bestowed upon certain rocks composed of 
a felsite-like ground-mass which has assumed a more or less schistose 
