434 J. D. Dana on some points in Lithology. 
In geology, it is essential to a thorough study of the ques- 
tions it has before it that the kinds of feldspars should not be 
massed under a common name; and that in every case the 
investigation should be considered unfinished unti] not merely 
the amount of silica in the rock is accurately ascertained, but 
also the particular species of feldspar is correctly and fully 
determined, however great the labor required to reach a conclu- 
sion. The use of the term plagioclase in such a case is an 
acknowledgment of incomplete work, and should be so treated. 
But the objection to the use of the term “plagioclase” is 
still stronger than has been stated. It now includes not only 
the soda-lime feldspars from anorthite to albite inclusive, but 
also part of potash-feldspar. The establishment, on an unques- 
tionable basis, of Breithaupt’s microcline by Des Cloizeaux, and 
his further observations that this triclinic potash-feldspar is a 
common mineral, much of what was supposed to be ortho- 
clase belonging to it, has extended the range of “ plagioclase,” 
until it is now almost an equivalent of the general term feldspar, 
so that “ plagioclase and hornblende” has, as to chemical consti- 
tution, the same signification now with feldspar and hornblende. 
6. Rocks consisting of a triclinic feldspar and mica.—The term 
dioryte, formerly defined as a rock consisting of oligoclase or 
albite and hornblende, has been introduced into the name ot a 
The remarks made respecting syenyte apply equally here; 
and also those respecting “ plagioclase.” A mica-dioryte is, like 
granite, eminently an alkali-yielding rock, the mica (biotite) 
affording usually ten per cent of potash ; and as granites often 
contain oligoclase as well as orthoclase, the amount of potash 
and soda in a “mica-dioryte” and a granite may not be very 
wade shaped Dioryte, on the contrary, is prominently a 
orn : 
to the mineralogical and chemical constitution of 
the rocks, we are naturally led to recognize along side of a 
mica and potash-feldspar series, which is headed by granite, 
also a mica and soda-lime feldspar series, and to include in the 
latter the so-called mica-diorytes. 
7. Hornblendie or Augitic—Hornblendie and augitic rocks 
stand apart as a general thing in all systems of lithology. Yet 
