ALTERATIONS OF STRATA. 
583 
trap into Lydian stone, a substance differing from horn¬ 
blende-schist almost solely in compactness and uniformity 
of texture.”* “In Shetland,” remarks the same author, “ar¬ 
gillaceous-schist (or clay-slate), when in contact with gran¬ 
ite, is sometimes converted into hornblende-schist, the schist 
becoming first siliceous, and ultimately, at the contact, horn- 
blende-schist.” In like manner gneiss and mica-schist may 
be nothing more than altered micaceous and argillaceous 
sandstones, granular quartz may have been derived from si¬ 
liceous sandstone, and compact quartz from the same mate¬ 
rials. Clay-slate may be altered shale, and granular marble 
may have originated in the form of ordinary limestone, re¬ 
plete with shells and corals, which have since been obliter¬ 
ated ; and, lastly, calcareous sands and marls may have been 
changed into impure crystalline limestones. 
The anthracite and plumbago associated with hypogene 
rocks may have been coal; for not only is coal converted into 
anthracite in the vicinity of some trap dikes, but we have 
seen that a like change has taken place generally even far 
from the contact of igneous rocks, in the disturbed region of 
the Appalachians. At Worcester, in the State of Massa¬ 
chusetts, 45 miles due west of Boston, a bed of plumbago 
and impure anthracite occurs, interstratified with mica-schist. 
It is about two feet in thickness, and has been made use of 
both as fuel, and in the manufacture of lead pencils. At the 
distance of 30 miles from the plumbago, there occurs, on the 
borders of Rhode Island, an impure anthracite in slates con¬ 
taining impressions of coal-plants of the genera Pecopteris^ 
NeuTopteTis^ Catamites^ etc. This anthracite is intermediate 
in character between that of Pennsylvania and the plumbago 
of Worcester, in which last the gaseous or volatile matter 
(hydrogen, oxygen, and nitrogen) is to the carbon only in the 
proportion of three per cent. After traversing the country 
in various directions, I came to the conclusion that the car¬ 
boniferous shales or slates with anthracite and plants, which 
in Rhode Island often pass into mica-schists, have at Wor¬ 
cester assumed a perfectly crystalline and metamorphic tex¬ 
ture; the anthracite having been nearly transmuted into 
that state of pure carbon which is called plumbago or 
graphite.f 
Kow the alterations above described as superinduced in 
rocks by volcanic dikes and granite veins prove incontesta¬ 
bly that powers exist in nature capable of transforming fos- 
siliferous into crystalline strata, a very few simple elements 
* Syst. of Geol., vol. i., pp. 210, 211. 
t See Lyell, Quart. Geol. Journ., vol. i., p. 199. 
