420 
CROSS. 
Excepting purely silicious forms of secondary origin, all 
the spherulitic bodies of these rocks are products of the con¬ 
solidation of a cooling magma. As an aid in appreciating 
some of the subsequent statements, it is thought advisable to 
give in the outset quantitative analyses of two typical pitch- 
stones, both in almost perfectly fresh condition, and repre¬ 
senting, as nearly as can be ascertained, the composition of 
two magmas in which spherulites were locally developed. 
Analysis I is of the pitchstone at Silver Cliff, in which occur 
the large compound spherulites to be described, while II rep¬ 
resents a small lava flow in the Rosita Hills. The analyses 
were made by L. G. Eakins. 
I. II. 
Si 0 2 ___ 71.56 73.11 
A 1 2 0 3 _ 13.10 13.16 
Fe 2 0 3 _ 0.66 0.62 
' Fe O_ 0.28 0.23 
Mn O_ 0.16 0.14 
CaO_ 0.74 0.54 
Mg O_ 0.14 0.19 
K 2 0_ 4.06 5.10 
Na 2 O_ 3.77 . 2.85 
H 2 O_ 5.52 4.05 
99.99 99.99 
Other analyses confirm these in showing the very simple 
constitution of the rhyolitic magmas of this district, each of 
which on complete crystallization must consist of nearly 
two-thirds alkali feldspar and a little more than one-third 
free silica. All other possible constituents are quite subordi¬ 
nate in amount, and it is to be noted that prior to the spher¬ 
ulitic period the lime, magnesia, and much of the iron oxides 
had been separated out in phenocrysts of plagioclase, leaves 
of biotite, microlites of augite, and in grains or trichites of 
magnetite. When the spherulites began to form in these 
lavas the molecular ratio of potash + soda to alumina in 
the still fluid part was almost exactly 1 : 1, and the only 
other important constituents were silica and an abundance 
of water. 
