33 
With an increase in the percentage of feldspar, the rock, on the average, 
becomes distinctly coarser. One peculiar and interesting phase is quite 
abundant. It consists of approximately 75 per cent feldspar, in the form 
of white or pink laths, varying in length up to an inch or more and in thick- 
ness up to one-quarter or even half an inch. These feldspar crystals stand 
in parallel arrangement and, on weathered surfaces, in many places exhibit 
zoning. The rock presents the trachytic texture characteristic of material 
that has solidified under differential pressure or during movement of the 
magma. The spaces between the closely crowded feldspar crystals are 
filled with a fine-grained mass of biotite and a dark green pyroxene. A few 
tiny specks of pyrite can be seen in the more basic varieties. 
A large part of the intrusive body consists of a coarse-grained rock 
made up of brownish feldspar crystals, varying in length up to 2 or 3 inches, 
with a very minor amount of white mica and a green, fibrous mineral 
between the feldspar grains. In some varieties there is a large amount of 
a fine-grained, grey matrix; in others there is practically none. 
The most acid phases occur as small dykes that cut all the above- 
mentioned varieties. The dykes seldom are more than a foot wide. These 
varieties produce some very beautiful rocks. One type is brownish white, 
weathers to a brick red, and shows numerous stubby laths of brown feld- 
spar set in a fine-grained, white groundmass. A few quartz crystals appear. 
Ferromagnesian minerals are essentially lacking. 
Numerous small dykes, none more than a few inches wide, consist of 
a fine-grained, grey rock that is essentially feldspar. Dark minerals, in 
greater or less amount, may or may not be present. In some cases, shreds 
of a blue-green amphibole have developed at or near the contacts of these 
dykes. This is particularly true where the dykes cut the more basic 
varieties of syenite porphyry. 
The final differentiation product appears to be represented by a few 
very coarse-grained dykes consisting of coarse crystals of feldspar and a 
considerable amount of quartz. They have an almost pegmatitic appear- 
ance. In one or two specimens of the more acidic types a few tiny specks 
of a brown mineral, taken to be garnet, were observed in the field. No 
garnet has been identified during the petrographic study in the laboratory. 
The minerals that have been identified in the syenite porphyry are: 
orthoclase, microcline, microperthite, plagioclase (predominantly albite), 
quartz, amphibole (three varieties), augite, aegirine-augite, aegirine, biotite, 
muscovite, apatite, magnetite-ilmenite, titanite, rutile (?), zircon (very 
sparingly), and pyrite. Alteration has produced calcite, epidote, chlorite, 
limonite, and kaolinitic material. 
The indices of refraction given below for several minerals were deter- 
mined by the immersion method, as outlined in Bulletin 679 of the United 
States Geological Survey, by Esper S. Larsen. The liquids used belong 
to the set in the petrographical laboratory of the Massachusetts Institute 
of Technology. The indices of the different liquids differ by about 0 * 006. 
Accordingly, by immersing a mineral grain in successive members of the 
set, its indices can be obtained accurately, in nearly all cases, to within 
0-003. The grains examined were taken from hand specimens of which 
thin sections have been studied. The other optical properties were ob- 
tained either from isolated grains or from thin sections. 
