Geological Survey 7th Annual Report
Obsidian Cliff, Yellowstone National Park
TABLE OF CONTENTS
Minerals composing lithophysæ
Trichites and microlites
Origin of fayalite and lithophysæ
Development of various structures in obsidian
Conditions modifying the development of lithophysæ
The cause of different layers of lamination
Geographical distribution of obsidian
IX. Southern end of Obsidian Cliff
X. Obsidian columns
XI. Cliff of lithoidal rhyolite
Fig. 1. Dense, radially fibrous, and concentrically banded lithophysa, with broad shrinkage cracks.
Fig. 2. Eccentric and branching lithophysæ.
Fig. 3. Lithophysa, with concentric shells, in black obsidian.
Fig. 4. Rose-like lithophysa.
Fig. 5. Dense, radially fibrous lithophysa, with broad shrinkage cracks.
(Figs. 1,2,4, and 5 are from the lithoidite of Obsidian Cliff and
are natural size; Fig. 3 is from the black obsidian and is twice
the natural size.)
XIII. Lithophysæ in the lithoidite of Obsidian Cliff, natural size
Fig. 1. Rose-like center surrounded by delicate rings which are
the bases of thin shells; the outer portion is massive and is
traversed by shrinkage cracks.
Fig. 2. Various forms of lithophysæ as they occur in the laminated lithoidite.
XIV. Varieties of lithophysæ
Fig. 1. Partially hollow, radially fibrous spherulite.
Fig. 2. The same, with parallel banded structure.
Fig. 3. Hollow spherulite, with concentric shells.
Fig. 4. The same, traversed by the flow structure of the matrix.
Fig. 5. Hollow hemispherule.
Fig. 6. The same, with parallel banded structure.
Fig. 7. Lithophysa formed at a gaping of layers of the rock.
XV. Sections of spherulites and granophyre groups between crossed
nicols, the principal planes of the nicols being parallel to the
edges of the plate
Fig. 1. Small blue spherulite, with a granophyre group of quartz and feldspar at its center; enlarged 40 diameters.
Fig. 2. Colorless, microscopic spherulite, showing irregular, dark cross; enlarged 153 diameters.
Fig. 3. Simplest form of granophyre group of quartz and feldspar; enlarged 235 diameters.
Fig. 4. Complex granophyre group of quartz and feldspar; enlarged 200 diameters.
Fig. 5. Granophyre group of quartz and feldspar in a rhyolite from Eureka, Nev.; enlarged 37 diameters.
Fig. 6. Augite microlites and magnetite trichites in black obsidian; enlarged 250 diameters.
XVI. Red obsidian in thin section
Fig. 1. Showing the contortions of the delicate streaks of yellow and brown glass within colorless glass; enlarged 60 diameters.
Fig. 2. Breccia of reddish-brown and colorless glass; enlarged 40 diameters.
XVII. Spherulitic structures in thin section between crossed nicols
Fig. 1. Spherulitic band through obsidian; enlarged 30 diameters. (The spherulites are the same as that of Pl. XVI, Fig. 2.)
Fig. 2. Branching feldspar fibers which are similar to the rays in the large, porous spherulites; enlarged 55 diameters.
XVIII. Spherulites in thin section
Fig. 1. Small, gray spherulites that have crystallized along a
line of microscopic spherulites which are so close together as to form a
continuous, fibrous band; others containing a belt of granophyre groups
of quartz and feldspar; enlarged 24 diameters.
Fig. 2. Larger, gray spherulite, with concentric banding, inclosing
smaller spherulites with brown margin; enlarged 20 diameters.
Fig. 3. Gray spherulite, partially developed; enlarged 20 diameters.
51. Crystal of soda orthoclase
52. Twinned crystal of soda
53. Crystal of fayalite
54. Crystal of fayalite
Last Updated: 22-Jun-2009