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[Assembly 
tinct and the line of junction between the adjacent portions well de- 
fined ^ as much so, as ever occur between a metaliferous vein and the 
rock embracing it. There are, however, many localities at which the 
limestone and granite are intimately incorporated, or irregularly mixed. 
They form together a rock subject to decomposition. Cavities of some 
considerable extent are frequently met with, which have been formed by 
disintegration and decomposition. They are often lined with crystals 
of calcareous spar, gypsum, and sulphate of baytes. Wherever lime- 
stone is incorporated or blended with granite in the manner here stated, 
the former is more or less charged with magnesia. 
In relation to the primitive limestone, there is one fact which I deem 
worthy of notice, and w^hich, it appears to me, has a bearing on the 
question under discussion. It is the presence of foliated plumbago in 
all the primitive limestones. At the first thought this fact may not ap- 
pear in point; but when it is compared with a result which always oc- 
curs in furnaces, when in good action, it certainly becomes important. 
It is interesting to observe the abundance of foliated plumbago in the 
slags of furnaces and forges. This substance is made when the furnace 
attains its maximum heat, and when it is in perfect operation, but never 
when the heat is below a certain standard, or when it works indiffe- 
rently. Now in both of the instances in which plumbago is found, may 
not its formation in both cases result from the same cause, viz : the ac- 
tion of heat, or in other words, as we know it to be produced by heat 
in the latter case, may we not infer that it is so in the former? Plum- 
bago is insoluble in water, and never has been formed artificially in the 
humid way; yet w^e know it may be formed by heat, though a combus- 
tible substance. I do not wish to make more of the facts than they 
are worth, standing as they do, they seem to favor the view here taken 
of them, and when corroborated by other evidence, the position assumed 
cannot be easily shaken. 
Again; I infer the igneous origin of the primitive limestones, from 
the effect which they have had on siliceous minerals with which they 
have come in contact; thus in Rossie, a vein of limestone in gneiss has 
rounded by partial fusion, apparently crystals of quartz upon its sur- 
face. Pieces of quartz, at the same place, in a massive state, have all 
the appearance of having been in a forge and partially melted. I do 
not confound in this case the slaggy appearance which results from a 
decomposition, with real igneous effects. Specimens illustrating the 
change I refer to, may be seen in the collection. The remarks I have 
made in the change produced in quartz might be extended to phosphate 
