240 Address before the Association of American Geologists. 



as described by Prof. Emmons, is one of the most interesting. 

 The same gentleman has, also, given us some remarkable details 

 respecting the occurrence of genuine injected veins of limestone 

 in granite, in the county of St. Lawrence. The facts have led 

 him to discuss the question whether all primary limestones ought 

 not to be classed among the unstratified rocks. This question, I 

 apprehend, we have in this country abundant means of deciding, 

 as we have the analogous question respecting serpentine ; since 

 we have numerous and extensive beds of both these rocks associ- 

 ated with the oldest of our strata. That they are metamorphic 

 in a high degree, no one can doubt : nor is it less certain that 

 serpentine connected with talcose slate and gneiss, exhibits nu- 

 merous divisional planes ; and often these are parallel to the 

 planes of stratification in the adjoining strata ; — but the question 

 still remains, whether that divisional structure may not be the 

 result of metamorphic agency instead of original deposition. 



The northwestern border of the primary stratified belt of rocks, 

 extending from Alabama to Canada, a distance of at least twelve 

 hundred miles, is composed of interstratified beds of taleose and 

 mica slates, gneiss, and granular limestone. I do not doubt (at 

 least, from all that I can learn) that these rocks are continuous over 

 this vast distance ; forming perhaps the longest belt of limestone 

 on the globe. A considerable part of this limestone is more or 

 less magnesian ; and in many places pure dolomite. It furnishes, 

 therefore, a fine field for studying the phenomena and the origin 

 of dolomitization. As to that portion of this field which has fall- 

 en under my observation, I find, that with one or two unimpor- 

 tant exceptions, all the cases of dolomitized limestone occur, 

 either in the vicinity of a fault, or of unstratified rocks, or of the 

 oldest gneiss. The pure dolomite is usually found where there is 

 reason to believe extensive dislocations of the strata occur ; and 

 the marks of stratification in the limestone disappear, nearly in 

 proportion to the amount of magnesia which it contains, so that 

 the pure dolomite shows scarcely any traces of it. I doubt not 

 that similar conclusions will follow an examination of other parts 

 of this deposit, so remarkably uniform is the geology of this 

 continent ; — and moreover, these conclusions correspond to the 

 history of dolomitization in Europe. They seem to render prob- 

 able the theory of sublimation from the interior of the earth. 



