PETROGRAPHY. 
109 
felted arrangement of the numerous augite prismoids. In part this is seen between 
crossed nicols to be feldspar both labradorite and to a less extent orthoclase in 
micropoikilitic patches, and in part an isotropic glass of about the same refractive 
index as the leucite, though usually slightly lower. 
In a general way this groundmass resembles in mode and fabric the vesbal 
type, though on a much smaller scale and with a much more pronouncedly tangen- 
tial arrangement of the augite prismoids. The effect, especially when the leucites 
carry few or no inclusions, is that of a net or section of a sponge, the felted mass of 
augite prismoids representing the threads or walls and the clear, colorless, round 
leucites the holes. This microtexture is so characteristic of many of the rocks of 
the region, and so easily recognizable that it is well-deserving of a special name. 
For this the term clathrate* may be used, borrowed from the glossaries of botany 
and zoology. It is, of course, conceivable that a similar textural habit may be pro- 
duced by the equant, spheroidal development of other salic minerals, as quartz, 
feldspar, nephelite, or the sodalites, along with the similar interstitial felt of 
augite or other alferric prismoids, though this is obviously not a very probable 
occurrence, and is as yet unknown as far as my knowledge extends, the texture 
being confined to leucite-augite rocks. 
As this habit, characterized by the development of leucite in round micropheno- 
crysts, surrounded by smaller and more or less tangential subhedral to anhedral 
augite prisms, is very common in the Roman Region and characteristic of many of 
the more femic rocks, it may well be given a special name. For this the term galeroid 
would seem appropriate. 
Chemical composition. An analysis of this type, published in incomplete 
form some years ago, is given below, with the addition of determinations of some 
of the minor constituents. The analysis of vesbal braccianose is also repeated for 
comparison. 
Chemical Composition of Galeral Braccianose [Leucitite]. 
I. 
II. 
I. 
II. 
SiO 2 
4.7 80 O 7o8 
4.7.6? O. 7O4 
CO 2 
none 
none 
ALO* 
17 8? 17? 
18 13 .178 
TiO 2 
o. 77 o. oio 
1 . 13 O.OI4. 
Fe 2 O 3 
4. 03 .O3I 
2.63 .017 
ZrO 2 
O.O2 
O.O2 
FeO 
3.64 . o?o 
6 . 48 . 090 
P 2 O, .. 
0.36 .003 
0.50 .004 
MeO 
3.68 .092 
4. IQ . IO? 
SO 3 
0.06 
trace 
CaO 
8 . 70 . i ? ? 
9.01 .161 
MnO 
n.d. 
n.d. 
Na 2 O 
2 60 . 042 
2.78 .04? 
BaO 
0.28 
0.24 
Ko 
H 2 + 
.23 .007 
0.65 
0.13 
99.68 
100.47 
H 2 O- 
O.II 
Sp.gr 
2. 781 
I . Galeral braccianose [leucitite]. West of Crocicchie, south of Lake Bracciano, Sabatinian 
District. Washington, analyst. Jour. Geol., V, 1897, p. 49. 
II. Vesbal braccianose [leucite-tephrite]. Lava of 1872, below Observatory, Mount Vesuvius. 
Cf. p. 104. 
* From Latin clathratus, latticed. 
