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 tliis 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 devclopement of the graphite, as yet seen only in small but 

 " very brilliant scales, which arc oftentimes hexagonal. Very soon the mass becomes mottled 

 " willi 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 devclopement 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 hi 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 mictuous 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 gi-aphite commences at temperatures lower 

 than 1500° Fahr., and roaches 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 sacchai-ine 

 limestones, has not been noticed by me. In this remark, I have reference to those limestones 



* Prof. RoDCKRs's Fmal Rtport «/ Atu-JcTsry, p. 74, 1810. t liipnri British Assiiciation, in the Alhcmeumfur 1839, p. 728. 



