LITHOLOGY. 159 
later, are 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. 
DioriTtz (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- 
