J, D. Dana on the Characters distinguishing Kinds of Rocks. 485 
the minerals are essentially identical in chemical composition, 
and related in crystallization, though different in their occurring 
crystalline forms and in the angle of the cleavage prism. 
identity in composition is so close that chemical analysis is not 
able to distinguish them. Hence the related eruptive rocks of 
the hornblendic and augitic series (or those containing the same 
species of feldspar in like proportions) must have originated in 
material of essentially the same chemical composition. The 
relation between the two minerals is thus far closer than between 
the triclinic species of feldspars. 
evertheless, too much importance is not given them when 
each is made distinctive of an independent series of rocks; for 
the very wide extent to which augitic rocks retain unvaryingly 
their augitic characters—such rocks constituting full two-thirds 
of the earth’s eruptive masses—shows that the special conditions 
producing augite, instead of hornblende, whatever they are, 
have often acted on a vast scale in the earth’s history. And 
So, also, the very wide distribution of hornblendic rocks, espe- 
cially among the metamorphic kinds, is evidence of a like com- 
peraneive influence of the conditions needed to make horn- 
lende in place of augite. The geological importance of the 
distinction is reason enough for recognizing it in lithological 
Systems. : 
8. Massive or Schistose.—Massive structure is often made prima 
facie evidence of igneous origin. Granite, with hardly a ques- 
tioning thought, has usually been placed solely among eruptive 
rocks. The igneous origin of dioryte even now is hardly left 
Open to investigation by some lithologists, Serpentine has been 
In the same category, though at present there are advocates of 
its metamorphic origin. And so other massive rocks are too 
likely to be set down as eruptive without a fair investigation. 
No two rocks are put farther apart in some lithological systems 
than granite and gneiss; and yet, none are more closely related 
in constitution and all essential characteristics. me 
The following are reasons for disregarding this distinction of 
Massive or schistose in classifying rocks, and for allowing a 
_ Massive structure little weight in deciding the question as to 
eruptive or metamorphic origin. : 
(1.) Massive rocks may be both metamorphic and eruptive. 
Granite, syenyte, with dioryte and other hornblendic rocks, are 
examples of massive rocks that are of both modes of origin. 
Many localities where kinds of these rocks occur metamorphic 
ave been described. I will mention two or three from the 
of granite, having a small northward dip, changes gradually 
to gneiss, and then to gneiss with some very micaceous mica 
