LITHOLOGY. 143 



were instituted, which now arc only employed in most special cases. To- 

 day, chemical analysis, with such ends in view, is only undertaken when 

 the necessity is indicated by the microscopic examination. 



But there is a value beyond the determination of mineral constituents, 

 which is attached to chemical analyses. Knowing the mineral constitu- 

 ents, it may be desired to determine their proportion; or, the ultimate 

 composition of a rock, without reference to its mineral constituents, may 

 be desired. On the data furnished by such analyses much reasoning has 

 been based, and many valuable conclusions arrived at, concerning the 

 chemical relationship of rocks formed at different times and in different 

 ways. The tabular works of J. Roth contain most of the analyses 

 which have been made. 



In the preparation of this work, some hundreds of thin sections of our 

 rocks have been prepared and examined, and chemical analyses have 

 been made, where an end seemed likely to be gained. It is hoped that 

 the specimens chosen as typical have been so carefully selected, that per- 

 sons can recognize, in the descriptions, the rocks from other localities in 

 the state which are not discussed, so that these pages may be found to 

 contain a tolerably complete presentation of the lithology of New Hamp- 

 shire. 



The following are the species of rocks that are considered in this re- 

 port, and the order in which they are described : * 



Diabase. 

 Diorite. 

 Gabbro. 



Basic Eruptive Rocks. 



Acidic Unstratified Rocks. 

 Felsitic. 



Felsite. 



Porphyritic Felsite. (Porphyries.) 



* In this cKissification only the most general divisions of the rocks are given, and the varieties and sub-varieties 

 are not enumerated. In the report, the extensive introduction of adjective terms in the nomenclature of varieties 

 and sub-varieties will be noticed. Many of these rocks have received special names ; but convenience and sim- 

 plicity are both promoted by making specific names dependent only upon the most fundamental distinctions. 

 The introduction of special names for rocks which possess particular local characteristics, or peculiar accesso- 

 ries, is not favorable to a science the characters of whose species are so very inconstant as are those of lithology. 



