LITHOLOGY, one 
blende and: pyroxene, which were made with the idea of indicating why 
hornblendic rocks are so predominant among the stratified schists in 
New Hampshire (see p. 65). Among these analyses one will be found 
which indicates the composition of the hornblende from the eruptive 
diorite at Dixville notch. A comparison of this with the pargasite will 
give the essential chemical differences between the hornblende of our 
two kinds of diorite. 
Some of the metamorphic diorites are free from quartz, and some in- 
clude quartz in their composition. On this basis, they are sub-divided 
into plagioclase diorites, which are compounds of plagioclase and horn- 
blende, with certain accessories, and quartz diorites, which contain quartz 
in addition, but not in preponderating amount. 
Plagioclase Diorite. These rocks are light green in color, and are 
granular and massive. Sometimes they form thick beds, and sometimes 
only thin strata among other schistose rocks. They are much less nu- 
merous than the quartzose varieties, and in composition they are very 
complex. Ina specimen from Pittsburg the hornblende is in such large 
grains that it can be macroscopically recognized. Thin sections show 
that this hornblende is of a light green kind, in which the dichroism and 
absorption are marked. As usual, the grains of hornblende are fibrous 
in their nature, and are all fringed out on the edges. In some grains 
which are cut at right angles to the prism, the characteristic cleavage is 
very plain; but other grains appear to be made of a mere mass of needles, 
and the fibrous structure prevents any such character as cleavage from 
being recognized. I have represented a section of this rock in Fig. 3 on 
Pl. 12, but have introduced only the most general features that character- 
ize it, and have left out the rare or accidental constituents. The mass of 
the white portion of the rock is a triclinic feldspar, which is characterized 
by the wide bandings that polarized light develops. These bands are 
often indistinct, on account of the decomposition to which the feldspar is 
very subject. As a result of this decomposition, epidote is a never-failing 
constituent of the rock. This epidote is light yellow, somewhat dichroic 
and gives most brilliant colors in polarized light. By its abundance, and 
: ? 
by the presence of multitudes of little hornblende crystals, and of other 
small minerals, the bands of color in the feldspar'are sometimes nearly 
obscured. Peculiar forms resulting from the decomposition of titanic iron 
