56 
GEOLOGY OF THE SECOND DISTRICT. 
“ derived from the elements of the blue limestone itself, which may easily be proved to con- 
“ tain an adequate quantity of iron and carbon for the production of this mineral.”* And 
again, p. 73, speaking of the gradation of the changes in this rock, the Report goes on to say : 
“We next behold a slight change of color to a lighter tint of blue ; and, at this stage of the 
“ alteration, we notice the first developement of the graphite, as yet seen only in small but 
“ very brilliant scales, which are oftentimes hexagonal. Very soon the mass becomes mottled 
“ with white, minutely granular carbonate of lime, the spangles of graphite growing progres- 
“ sively larger. Approaching still nearer to the dyke, the whole rock assumes the white 
“ sparry character, and contains, near the line of contact, besides graphite, several of the 
“ numerous crystalline minerals of the vein itself.” 
The slight difference in the views of Prof. Rodgers and myself, as it regards the question 
of the origin of the limestone, has no unfavorable bearing upon the question. The Professor 
maintains the developement of plumbago by igneous action upon a limestone. My views of 
the limestone differ from his, but not of the causes by which the graphite has been produced. 
The observation of the graphite in the slags of furnaces, differing in no respect from that of 
the mineral in rocks, led me to make the suggestion I did in the Report for 1838, that the 
foliated plumbago might also have been produced in the limestone in a mode very analogous 
to that observed in furnaces, knowing very well that all who were conversant with chemis¬ 
try, and with the composition of limestone, would admit its probability. 
It would be interesting to examine the composition of graphite as it occurs in rocks, in order 
to test the question whether there are two compounds of this substance, as it appears there 
are when it is produced in the artificial way in furnaces. According to the researches of Dr. 
Charles Schafhaeutl of Munich, there are two kinds or sorts of graphite, which may be pro¬ 
duced by running fluid puddling slag, or silicates of iron and manganese, over fragments of 
pit coal. One is in elastic scales of the thickness of writing paper, with rather a dull metallic 
appearance ; the other is of the thickness of gold leaf, and extremely unctuous to the touch. 
The first is a silicate of iron and carburet of silicon; the other, carburet of iron and carburet 
of silicon. It farther appears that the formation of graphite commences at temperatures lower 
than 1500° Fahr., and reaches its highest point not much exceeding 2000°.t 
From all the researches which have been made of graphite, it appears to be highly probable 
that the only method of forming it is by the action of caloric on some substance which con¬ 
tains its elements; and being a mineral so constantly present in this variety of limestone, its 
presence of itself makes it exceedingly probable that it has been developed in the rock by 
igneous action. If so, it is one step towards the solution of the problem concerning the origin 
of the mass which contains it. This view is entirely independent of the question whether 
the limestone was originally an igneous product like granite, or is a metamorphic rock accord¬ 
ing to the opinions of Profs. Rodgers and Hitchcock; for, by either view, its formation is by 
the same agency. As yet its occurrence in what have been usually considered saccharine 
limestones, has not been noticed by me. In this remark, I have reference to those limestones 
Prof. Rodgers’s Final Report of Neiu-Jersey, p. 74, 1840. 
t Report British Association, in the Athenmum for 1839, p. 728. 
