INTRODUCTION. 



Notwithstanding the extensive outcrops of all these calcareous strata in the 

 Northern Appalachians, there are few extensive deposits of calcareous matter from 

 springs, and far less than occur in the unaltered limestone districts. The thermal 

 springs of the western slope in New- York, which evolve nitrogen, produce no 

 mineral deposits of any kind. 



The imperfect study of these metamorphic rocks which I was able to make, 

 while engaged in other duties, during the years 1843, 1844 and 1845, enabled me 

 to recognize among these deposits a certain order which appeared to me to be 

 marked by the presence of characteristic minerals which had been segregated from 

 the surrounding mass. These observations, combined with subsequent considera- 

 tions of the subject, convinced me that much might be done in the recognition of 

 metamorphic masses by the contained minerals ; and that a proper study of these 

 would reveal some means of identification of beds at distant points, analogous to 

 the mode of distinguishing successive formations by their contained fossils. I have 

 subsequently, on many occasions, advocated this view, in discussions before the 

 American Association for the Advancement of Science, and I am convinced that 

 something of this kind will yet grow out of the farther investigation of the meta- 

 morphic rocks and their contained minerals*. 



In approaching the study of the metamorphic masses from the non-metamorphic 

 rocks, or, as we might almost say, from the organic side, they impress the student 

 very differently from the same rocks, if first studied in all their variety as mineral 

 masses, and designated as they are by certain names which are either arbitrary 

 or derived from the results of metamorphic action ; for example, gneiss, mica 

 slate, talcose slate, hornblende slate, calcareous mica slate, etc. etc. : names con- 

 trived to express the mineral condition of the rock, and the prevailing minerals 

 of which it is composed. 



The student from the unaltered rocks has been accustomed to see all the sedi- 

 mentary strata presenting the aspect of fine shale or slate, or of sandstone and of 



roborate what was before claimed, but bring to light some facts which indicate a greater extension of 

 the Devonian rocks to the westward than we have been accustomed to believe. 



• Every observing student of one or two years' experience in the collection of minerals in the New- 

 England States, well knows that he may trace a mica schist of peculiar but varying character from 

 Connecticut through Central Massachusetts, and thence into Vermont and New-Hampshire, by the 

 presence of staurotide and some other associated minerals, which mark with the same unerring 

 certainty the geological relations of this rock, as the presence of Pentamerus ohlongus, P. galeatia, 

 Spirifer niagartnsii, or S. jnaeropleura, and their respectively associated fossils, do the relations of 

 the several rocks in which they occur. 



