LAMINATED PYROCRYSTALLINE ROCKS. 
101 
to ring like metal when struck with a hard body; but it has 
not become gneiss or hornblende. The extent of all the indi¬ 
vidual rocks under consideration; together with their identity 
of structure the world over, is to say the least, indicative ol 
their early consolidation, and that it took place at a period 
when water existed only in a vaporous state. 
These remarks, it will be perceived, apply to this country. 
The metamorphic gneiss of the Alps is admitted by eminent 
geologists. It is, of coarse, local, and we can not set bounds 
to extent of local changes. Still the metamorphic gneiss of 
the Alps originating in oolite and eocine rocks, must furnish 
by analysis, a difference in the proportion of their elements from 
that which exists in our normal gneiss or mica slate. It is now 
admitted that a parallel structure of itself is no evidence that 
the rock was a sediment. I pointed out, in my New York Geo¬ 
logical Reports, that the porphyry of lake Champlain was 
laminated, and described it as stratified rock, notwithstanding 
the indubitable evidence it furnishes at the locality, that it 
was erupted from fissures in the shales of the Hudson River 
group. Darwin describes an eruptive red granite of Chili, 
which exhibits a decided parallel structure in many of its parts. 
The gneiss of Bahia, according to the same author, contains 
regular fragments of hornblende; hence gneiss may be regarded 
as a pyrocrystalline or eruptive rock, at many localities. We 
should subject the question of metamorphism to two tests: 1. 
Proximity to agencies competent to effect the change observed. 
2. The continuity of the changed to the unchanged mass. 
The only proof, therefore, which we can obtain, of meta¬ 
morphism, is in the local change which may have been pro¬ 
duced in a part of the rock. There will then be gradations, 
which may be traceable from zones of the greatest to zones of 
the least change. We may trace the harder, ringing, reddened, 
or whitened mass, to those parts which retain their original 
properties. Beyond this we can not go. To this extent it is 
useful to admit the metamorphic theory. 
Gneiss is somewhat variable in structure and composition. 
