CONSTITUTION AND ORIGIN OP SPHERULITES. 421 
All substances older than the spherulites are arranged to 
bring out the fluidal structure of the viscous magma. Here, 
as elsewhere, this structure traverses the spherulites without 
change in course, but the earlier mineral particles have been 
more or less attacked by the agencies active in the spheru- 
litic periods. This action reaches a maximum when the 
microlitic crystals are wholly resorbed and the fluidal struc¬ 
ture is thus obliterated. 
Extremely minute microscopic spherulites were the first 
to form, although also developed in later periods, and several 
varieties may be distinguished among them. Some are 
clearly micropegmatite groups ; others are too dense to show 
any definite intergrowth of minerals. The early ones are 
usually colored brownish, but often have an outer colorless 
zone, plainly contemporaneous with adjoining spherulites 
which are colorless throughout. Dense cores are occasion¬ 
ally surrounded by an outer zone of later date, composed of 
distinct feldspar rays, and, rarely, microspherulites of appar¬ 
ently pure feldspar are found. 
Globules of opal or chalcedony are sometimes present in 
these rocks, chiefly in certain bands of lithoidal rhyolite, in 
druses, or in veins. They seem referable in all cases to the 
last period of spherulitic growth, or to a definitely secondary 
period. No reason has been found to suppose that purely 
silicious spherulites formed here in any period when the 
magma yet contained the components of feldspar in con¬ 
siderable amount. 
Further description of these minutest spherulites is un¬ 
necessary, as they seem to correspond to varieties developed 
in unusual perfection in the lava of Obsidian Cliff, which 
have been described by Iddings in the publications already 
cited and in an article read before the Philosophical Society 
at the same meeting with the present one. Iddings shows 
conclusively that a very large share of those dense micro¬ 
spherulites affording a distinct, negative interference cross 
must be supposed to consist of micropegmatite. This is a 
most natural product of crystallization of such magmas, and 
