Address before the Association of American Geologists. 241 



I have 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 

 Pennsylvania and Yirginia. 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 produce this result. 

 Farther examination, in other localities, will probably throw addi- 

 tional light on the subject. 



The phenomena of dykes and veins, especially along the east- 

 ern margin of the primary ranges of New England, 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, traced two of these, through gneiss and mica slate, 

 nearly across the state of Connecticut ; and I ihink I have found 

 their continuation across the whole of Massachusetts ; nor do I 

 doubt that they extend far into New Hampshire, Already these 

 two dykes have been followed nearly ninety miles in length ; and 

 they are usually several rods wide. Their direction almost coin- 

 cides with the strike of the strata. In Maine, they have been 

 found in great number and extent, by Dr. Jackson, in the primary 

 strata, and they have more distinctly the character of genuine 

 dykes than in Connecticut and Massachusetts. 



In some of our sienitic rocks, we find a perfect plexus of dykes 

 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 

 another proves the posteriority of the latter. The dykes at this 

 spot are varieties of greenstone, and the veins chiefly feldspar. 



Among the multitude of substances in our primary rocks that 

 deserve further attention, I can here mention only the oxide of 

 tin. Three localities of this mineral, affording however only 

 small disseminated crystals, have long been known in Massachu- 

 setts. But within the last year, Prof Shepard has discovered a 

 more promising locality in Connecticut, and Dr. Jackson another 

 in New Hampshire. The probability is, therefore, strong, that 



Vol. xLi, No. 2.— July-Sept. 1841. 31 



