LAMINATED RYROCRYSTALLINE ROCKS, 
103 
soft. Varieties occur in which the quartz is the principal 
mineral, and its structure then resembles a fine-grained sand¬ 
stone. On the other hand, when talc predominates it becomes 
steatitic or a perfect steatite. Mixed largely with scales of 
mica, it becomes a talco-micaceous slate. Like other rocks 
of this epoch, it passes into one or the other mineralogi- 
cal mass, with the necessary exchange or substitution of the 
mineral which characterizes them. It is associated with horn¬ 
blende rather than mica slate. 
§72. Hornblende rock. Its color is green, light green, or 
blackish green, and its composition is either an unmixed horn¬ 
blende, or else it is mixed with feldspar and quartz, the.parti¬ 
cles of each being arranged in parallel bands. In this last 
particular it differs from sienite. Hornblende is exceedingly 
tough, and the mineral is always crystalline. The crystals are 
interlaced with each other. Its composition is variable. It 
preserves, however, a great uniformity of character when asso¬ 
ciated with other laminated pyrocrystalline rocks; it is more 
variable in a trapean region. 
The laminated pyrocrystalline rocks lie in proximity to each 
other. It is rare for a mountain to be composed exclusively of 
one of these rocks, and it frequently happens that gneiss, mica 
slate, and hornblende form an alternating series, in which they 
are separated by short distances only. On the sides of mountains, 
and in valleys, their planes of lamination incline steeply to the 
horizon, while perhaps upon the crests of high ridges the 
laminae are nearly horizontal. This seems due to an upward 
thrust, by which the upper parts of the rock being unsupported, 
fall into an horizontal position. At the point of flexure the 
mass is frequently broken, when the lower portion of the rock 
is left highly inclined to the horizon, and the broken part is 
nearly prostrate. Such is the position of the talcose slate of 
Table mountain in Burke county, North Carolina (fig. 19). The 
body of the mountain is composed of strata highly inclined to the 
west, but the summit is quite flat. The porphyritic gneiss of 
the Swannanoegap of the Blue ridge, in North Carolina, is 
