Vol. 8, 1922 
PETROLOGY: H. S. WASHINGTON 
323 
The general absence of titanium and manganese is to be noted, while chro- 
mium is present in easily determinable amounts in all the specimens in 
which it was looked for, except No. 1. Considerable chromium is present 
in some of the purely gray specimens, as No. 6), which shows no trace 
of green color. 
The analyses are arranged in the order of decreasing pyroxene (diopside- 
jadeite) and increasing feldspar. The density varies quite regularly with 
this, but it is also influenced by the amount of jadeite relative to diopside 
in the pyroxene, those higher in jadeite showing the higher density. The 
relative amounts of jadeite and diopside vary widely and without any 
very marked regularity. 
The refractive indices of some of the pyroxenes, determined by Dr. 
Merwin, are given in table II In this the relative amounts of jadeite 
and diopside (calculated to 100%) are given, the amount of albite (and 
anorthite) held in solid solution in the pyroxene being neglected. The serial 
progression in the values for a and 7 as well as for the birefringence 
7 — 0:,^ as the pyroxene changes from nearly pure jadeite to pure diopside, 
is evident. There are slight irregularities at the extremes, and it would 
appear that the influence of the "occult" albite is simply to lower the re- 
fractive indices. This matter is to be studied further by Merwin and me. 
TABLE II 
RBPRACtivE Indices of the; Diopside- J adeitb SBRi:es 
Jd 
Di 
OL 
7 
y — a 
Jadeite (Tibet)^ 
98.68 
1 
32 
1.655 
1 
.667 
0.012 
Chichen Itza (No. 
6) 
95.45 
4 
55 
1.651 
1 
.667 
0.016 
Chichen Itza (No. 
4) 
93.69 
6 
31 
1.650 
1 
.668 
0.018 
Chichen Itza (No. 
2) 
87.62 
12 
38 
1.655 
1 
.672 
0.017 
Chichen Itza (No. 
3) 
77.89 
22 
.11 
1.660 
1 
.678 
0.018 
Tuxtla (No. 1) 
46.60 
53 
.40 
1.666 
1 
.688 
0.022 
Copan pebble (No 
. 11) 
28.45 
71 
55 
1.665 
1 
.693 
0.028 
Diopside (artificial)^ 
none 
100 
00 
1.664 
1 
.694 
0.030 
The feldspars show much less variation, which is not quite regular, 
but in general the feldspar becomes more albitic with increase of the 
amount of feldspar in the specimen. From what has been said, it is evi- 
dent that the pyroxene of the wholly pyroxenic specimens contains con- 
siderable "occult" feldspar, both albite and anorthite, held in solid solu- 
tion. No. 5 shows no feldspar in the thin section, although about 18% 
of AbseAun must be calculated as present. No. 6, however, which con- 
tains about 23.46% of similar normative feldspar, shows a few per cent 
present in the section. From a somewhat cursory study of the sections 
of the analysed specimens from this on to No. 12, it appears that the 
amount of visible (modal) feldspar is uniformly less than the calculated 
