Address before the Association of American Geologists. 241 
Thave noticed another analogous and singular fact in connec- 
tion with this limestone, and doubt not that it is common through- 
out its whole extent ; although I have seen it mentioned by no one 
except Prof. Mather, in his account of the rocks of eastern New 
York; but am informed by Prof. Rogers that it is common in 
Pennsgirdnia and Virginia. Where the limestone comes in con= 
tact with mica and talcose-slates, they are often highly impregna- 
ted with carbon, for several feet or rods from the line of junction, 
There can hardly be a doubt that the carbonic acid, which has 
penetrated the slates, has been decomposed to pusdiden, this result. 
Farther examination, in other localities, will probably throw addi- 
tional light on the saabjacts 
‘The phenomena of dykes and veins, especially wend the east- 
em margin of the primary ranges of New Bogland, are of a 
highly interesting character. Some of the dykes of greenstone 
appear to be of great size and extent. Dr. Percival has, with 
great labor, spree cence vathese; ‘through gneiss and mica slate, 
and Ithink I dateres found 
- 
tat. me ; a} 7 ‘ 
& ee 5 nor do &: 
doubt that they extend far into Ne wv Hampshire. ae thoea 
two dykes have been followed nearl P ¥ miles in Jeng th; and 
they are usually several rods Wides: Their. direction: 
cides with the strike of the strata. In Maine,. shies hawnchess: 
found in great number and-extent, by Dr: Jackson, in the primary. 
Strata, and they have more distinctly the abbas of. stot 
dykes than in Connecticut and Massachusetts. _- 
In some of our sienitic rocks, we find a perfect Pron of dries 
and veins. I have examined one spot in the city of Salem with 
a good deal of attention, and: I cannot-see why it does not afford 
Us evidence of the protrusion of. unstratified rocks at eleven dif- 
ferent epochs; admitting ‘that the intersection of one vein by 
‘proves the posteriority of the latter. The dykes at this $ 
Spot-are varieties of greenstone, and the veins chiefly feldspar...» 
“Afong the multitude of substances in our primary rocks that 
deserve further attention, Ican here mention only the oxide of 
tin... Three localities of this mineral, perme however. only 
small disseminated crystals, have long been known in 
Sets. “But within te as ye ‘Prof. Shepard: has. discovered a 
in New Hampshire. ph 
Vol. xt1, No. 2.—July-Sept. 1841. 
. 
a, 
