Geological Survey 7th Annual Report Obsidian Cliff, Yellowstone National Park

Cover

Introduction

Geological occurrence

Lithological structure

Columnar cracking
Lamination

Petrographical character

Obsidian
Spherulites
     Hollow spherulites
Lithoidite
Lithophysæ
     Minerals composing lithophysæ
          Quartz
          Tridymite
          Feldspar
     Fayalite

Microscopical characters

Trichites and microlites
Granophyre groups
Spherulites
     Porous spherulites
Fayalite

Origin of fayalite and lithophysæ

Mineral association
Chemical evidence
Conclusion
     Apparent exceptions

Development of various structures in obsidian

Conditions modifying the development of lithophysæ
The cause of different layers of lamination

Historical review

Geographical distribution of obsidian

Conclusion

PLATE

IX. Southern end of Obsidian Cliff

X. Obsidian columns

XI. Cliff of lithoidal rhyolite

XII. Lithophysæ

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.

FIGURE

51. Crystal of soda orthoclase

52. Twinned crystal of soda orthoclase

53. Crystal of fayalite

54. Crystal of fayalite