LITHOLOGY. 159 



later, arc very nearly related to diabase, and, indeed, may well be classed 

 as varieties of it. They have, however, such distinctive characters that 

 I do not like to introduce them here between rocks which are more in 

 need of classification. 



DiORITE (PORPHYRITE). 



Diorite is a crystalline, granular mixture of a triclinic feldspar, horn- 

 blende, and an oxide of iron, which is either magnetite or titanic iron. 

 There are in New Hampshire two well defined and very distinct kinds of 

 diorite. One contains a green, more or less fibrous, hornblende, and also 

 often contains quartz, which at times is present in such amounts as to 

 relate the rocks to the amphibolites in composition. The rocks are light 

 green in color, and though massive their beds are arranged conformably 

 with the surrounding strata. Such rocks in Canada have been shown by 

 Dr. Hunt to be sedimentary beds metamorphosed into diorites. The 

 conformity of rocks of this nature at New Haven with the surrounding 

 strata has been clearly shown by Prof. Dana ; and others have followed 

 in their studies upon rocks of various regions. The diorites of the other 

 class are in New Hampshire black rocks. The hornblende that they 

 contain is in compact grains, or in crystals with defined outlines. It is 

 not green, but is black, and in thin sections it is deep brown or dark 

 yellow, and is strongly dichroic. The rocks of this nature occur in well 

 defined dykes cutting through the strata, and are plainly eruptive. It 

 is understood that, in this place, we are treating only of the latter class; 

 the former are described among the greenstones of the Connecticut 

 valley. 



These rocks sometimes resemble diabase, and though in those that 

 contain large crystals the hornblende can be recognized by its cleavage, 

 in the more compact varieties this is not possible. A glance with the 

 microscope is, however, sufficient for their determination ; and the neces- 

 sity for careful discrimination by such a method becomes very evident 

 when it is found that dykes of diorite and diabase are situated side by 

 side, specimens from which could with difficulty be distinguished from 

 one another. Being so associated, little room is left for generalization 

 upon their relationship to one another as regards position. The micro- 



