156 E. P. CAREY AND W. J. MILLER 
the most striking features of the coarser-grained specimens is the 
prominence of the hornblende crystals which make up one half of the 
rock. Because of slower cooling, the inner portions of the large dikes 
are more coarsely crystalline than the outer portions, while the narrow 
dikes and numerous branching tongues are fine-grained. 
Upward of twenty thin sections were examined, and the chief 
constituents were found to be: plagioclase, hornblende, augite, 
biotite, and iron-oxide. Plagioclase, chiefly oligoclase and andesine, 
is, in all cases, the most abundant mineral. It usually occurs in 
large idiomorphic crystals, frequently twinned according to the albite 
law. ‘The crystals are always weathered, sometimes almost beyond 
recognition, especially in the older set of dikes. 
Hornblende, rather evenly scattered through the sections, is almost 
as important as plagioclase. It shows its characteristic pleochroism, 
and in many cases also its typical cleavage angle of about 124°. 
Sometimes the hornblende is concentrated into small green patches 
about the size of a pea. 
Orthoclase is a minor constituent compared to plagioclase and 
usually occurs in irregular grains. 
Augite is quite abundant in some specimens, while in others it 
is entirely absent. It usually appears in well-developed crystals 
showing a brown color, very slight pleochroism, and characteristic 
cleavage angles of 87°. 
Biotite is a minor constituent, but often occurs in the typical 
brown patches with fine basal cleavage and strong pleochroism. 
Iron oxide occurs as occasional opaque irregular grains in all of 
the sections examined. 
A study of the diorites shows a considerable variation of these 
rocks among themselves. Some contain very little augite and feldspar 
while they are very rich in hornblende. Such rocks would readily 
suggest a similarity to the gabbros in basicity. Other diorites con- 
tain much less hornblende, but more feldspar and augite, and are 
the most acid rocks of the series. 
Chemical analysis.—The following is a partial analysis of the 
typical Quarry Hill diorite furnished by the United States Geological 
Survey to. Dr. J. C. Branner: 
