Modern Igneous Rocks and the Primary Formations. 105 
I shall endeavor to establish 
Ist, That the schistose acquaint gneiss: ‘and mica iad, i is no 
satisfactory evidence of a sedimentary origin ; 
2d, That some granites with no trace of a schistose structure, 
may have hada sedimentary origin 
3d, That heat producing the aia that are sessed metamor- 
phic, was not applied from beneath by conduction from some in- 
ternal source of heat; on the contrary it was applied through the 
waters of the ocean, covering and permeating the deposits which © 
received their high temperature from the eruption itself. Tn oy 
other words, the metamorphic rocks so called are not hypogene, 
as explained by Mr. Lyell, but—to use corresponding phraseol- 
ogy—epigene, or analogous to other rock RRS a 
and solidified on the surface of the earth. 
_ The argument for the sedimentary origin of gneiss, mica slate, 
etc., is based upon the assumption that known igneous rocks do 
not assume a schistose structure. But this is far from true, for the_ 
descriptions of most volcanic regions mention the occurrence of 
laminated trachytes, basalts and porphyries. It is by no means 
unusual to find basalts and basaltic lavas with parallel lines of 
lamination, sometimes appearing only after being weathered, and — 
at others so distinct as to admit of easy cleavage. This takes pla ec 
| both if massive and columnar basalt. At the Cape Verds, onthe 
| shores just below the town of St. Jago, the columns are gradually 
falling to pieces, owing to an exfoliation of the summit, from 
which curved plates separate easily, usually from a fourth to half 
an inch thick. Such instances, which are not uncommon, are 
imputed to a concentric structure. This is no doubt true, but the 
concentric structure is but one mode of crystallization, and crys- 
tallization as we believe, is the cause of the schistose structure in 
all igneous rocks. Massive. basaltic lavas splitting into aes 
lamin an inch or so'thick, are met with at the Sandwich Islands. — 
Laminated or slaty trachytes are too well known to require more 
| than a mere allusion to them. “The structure is far more thinly ~ 
schistose than any gneiss, and often nearly as much so as many 
mica schists. Laminated porphyry is described by Prof. Em- 
mons as occurring at Cannon’s Point in northern New York, 
which splits into plates from a fourth to an inch in thickness. 
- A schistose structure then is certainly no evidence that the rock 
was not originally igneous; and if we consider how exactly this 
Vol. xiv, No. 1.—April-June, 1843. 14 
