AN ANALCITE-BASALT FROM COLORADO 689 
TABLE OF ROCK ANALYSES. 
TVie Vi VI. VII. 
Analcite-basalt, Monchiquite, Monchiquite, Plagioclase-basalt, 
The Basin, Colo. | Shelburne Point, Vt. Brazil. Saddle Mt., Colo. 
W. F. Hillebrand. H. T. Vulté. M. Hunter. W. F, Hillebrand. 
SiO, 45.59 45.58 46.48 48.76 
TiO, T32 undet. .99 1.65 
FiO) 03 76 Oh Stee none 
Al,O; 2208 15.87 16.16 15.89 
Fe,0, 4.97 4.65 6.17 6.04 
FeO 4.70 6.37 6.09 4.56 
MnO A trace meres 1 
CaO II.09 9.91 Teas 8.15 
SrO ae Tee: peseks .06 
BaO a) Bestel SHG cil 77 
MgO 8.36 8.32 4.02 5-98 
K,O 1.04 1.61 3.08 2.93 
Na,O 4.53 3-42 5.85 3-43 
Its (O) trace eens sisters none 
H,O yi ) ~ 1 ( .40 
H,O 3.40 J oa S ot) / 1.48 
P.O; OI rity eyes .60 
Cl 05 Res cette 
Cor Ser 45 
99.87 98.87 100.91 100.23 
Were the greater part of the water in the analcite-basalt 
deducted, and the remainder recalculated to 100, the analysis 
IV might stand for an ordinary nepheline-tephrite. It is lower 
in alumina than many analyses of basalt, but if TiO,, P,O,, 
and MgO were accurately estimated in all such rocks the alu- 
mina would often fall 2 or 3 per cent. below the published figures. 
The magma of this basalt was relatively quite rich in soda with 
low silica and much water. The formation of analcite in the 
final stages of consolidation of such a magma seems to me much 
more natural than that glass should be the result, provided only 
that the conditions were in general favorable to crystallization. 
As the rock probably occurs in a dike and ina region where 
there are many dikes of holocrystalline plagioclase-basalt, the 
presumption must be that the conditions were favorable to crys- 
tallization. 
