44 J. P. Iddings—Nature and origin of Lithophyse, etc. 
this Journal in May, 1886 (page 325.) It was there considered 
to be of different origin from the tabular parting produced by 
rapid cooling near the surface of a body of molten rock. 
The origin of the more general lamination which in one form 
or another usually extends throughout the whole mass of many 
lava flows will be readily understood from the following: 
In a fluid free to flow over a horizontal surface the movement 
of its molecules will meet with least resistance in directions 
parallel to the plane of that surface, the fluid will therefore 
spread horizontally, and its molecules will move in planes 
parallel to the underlying surface. Particles suspended in the 
fluid will be carried along these planes, and portions of the 
fluid which contain different amounts or different kinds of sus- 
pended matter will be spread out in layers along these planes of 
flow. 
In voleanic lavas the production of such layers will depend 
on the lack of homogeneity and viscosity of the magma, and 
the distance over which the lava flows. The more basic lavas 
are usually more liquid and consequently more homogeneous 
at the time of eruption, and show little if any signs of layer- 
structure in the solidified rock. But acid lavas such as rhyolite 
appear to be more viscous and less homogeneous when erupted, 
and slight variations in the consistency or composition of the 
mass show themselves as bands and streaks of colors or as lay- 
ers of differing microstructure and degree of crystallization. 
These layers of different consistency were probably lenticular 
or quite irregularly shaped portions of the magma near the 
place of its eruption, and are more perfectly and thinly lamina- 
ted the farther from the source. 
The nature and cause of such local differences in the magma 
is suggested by a consideration of the structure of the various 
layers of the rock forming Obsidian Cliff. The lithoidal rock 
presents layers which differ in their degree of crystallization, 
some being wholly uncrystallized and glassy. Some are finely 
spherulitic, others coarsely so and porous; and others are quite 
granular and full of cavities. In the obsidian the differences 
find expression in layers of spherulites, and bands of litho- 
phys; in layers more or less rich in granophyre feldspars, 
microspherulites, microlites and trichites; that is, in the dit- 
ferent phases and amount of crystallization developed. Nearer 
the surface of the obsidian flow there is less crystallization, 
but the lamination of the rock is quite as noticeable in the 
more marked differences in the amount of microlites in the 
various layers and in the abundance of gas cavities which pro- 
duce alternating bands of vesicular and dense glass and pumice. 
While at the surface of the flow the whole rock | is openly pu- 
miceous. 
