The leaf. The leaves of the rosette are thick, fleshy, and glabrous; they are held in an almost vertical position,

and the stomata are equally distributed over both faces of the blades. While cuticular striations are very pronounced on the ventral face of the leaf, (fig. 10) the cuticle

is smooth on the dorsal. Viewed "en face" the lateral cell-walls of epidermis are, on both faces, straight, quite thick and porous, especially on the dorsal face; the stomata (fig. 8) are free, and surrounded by mostly four ordinary epidermis-cells. In cross-sections the structure of the cuticle is readily visible (fig. 10), and the outer cellwall of epidermis appears quite thick. The chlorenchyma consists of four layers of palisade-cells, which cover a very open pneumatic tissue of about eight strata. In the marginal portion of the leaf the palisade-tissue extends to the dorsal part of the blade and there is no mechanical tissue neither stereome nor collenchyma in any part of the leaf. The mestome-strands are collateral, all single, and surrounded by green parenchyma sheaths. The leafstructure is thus approximately isolateral on account of the distribution of the stomata, but not with reference to the structure of the chlorenchyma. The leaf-structure thus resembles that of the high alpine Claytonia megarrhiza, as far as concerns the distribution of the stomata on both faces of the blade, the several strata of palisadecells, the thick cuticle and thickwalled epidermis. Bonniers and Wagner' have offered some interesting contributions to the knowledge of the structure of alpine plants, and according to these authors the leaf-structure of Chionophila and of Claytonia megarrhiza appears to be the typical one, which characterizes the overwintering leaves of alpine species. And the highly developed assimilating tissue is in correlation with the pronounced intensity of light at the higher elevations, the considerable decrease of carbonic acid in the atmosphere, beside the very short time of vegetation. At the same time these winter-green leaves are protected against too excessive transpiration during the period of the melting of the snow, by means of the very thick cuticle and pronounced thickening of the cell-walls of epidermis. However, only the leaf has been studied by these authors and the Monocotyledones have,

Holm, Theo. Claytonia Gronov, Mem. Natl. Acad. of Sc., vol. 10, Washington.

Bonnier, G. Cultures expérimentales dans les hautes altitudes, (Comptes Rend. Acad. Sc., 1890).

Bonnier, G. Influence des hautes altitudes sur les fonctions des végétaux, (ibid. 1890).

Bonnier, G. Etude expérimentale sur l'influence du climat alpin sur la végétation et les fonctions des plantes, (Bull. Soc. Bot. de France, 1888).

Wagner, A. Zur Kenntniss des Blattbaues der Alpenflauzen und dessen biologischer Bedeutung, (Sitzungsber. Akad. Wiss. Wien, Math. nat. classe, May, 1892).

almost, been passed by in silence. To obtain a more complete idea of the alpine structure and its correlation with the surrounding medium it will be necessary to include the stem as well as the root structure, and to decide whether certain structural peculiarities may be really epharmonic or inherited.

Finally with reference to Pentstemoniopsis this plant shows exactly the same monopodial growth as Chionophila: a terminal rosette of leaves, axillary floral stems, and axillary stolons. The monochasium is also represented in this plant, and more distinctly than in Chionophila, since the inflorescence is more open. The seeds also agree with those of Chionophila (fig. 15), but the flower and the fruit resemble those of Pentstemon. The habit of the plant thus corresponds with that of Chionophila, the deeply cleft calyx and the free capsule with those of Pentstemon. The corolla with the lower lip bearing short-papillae instead of distinct hairs as in Chionophila is of small importance, but the gibbous base, and the horizontal position of the flower render the plant distinct from both, and especially when combined with the monochasial ramification of the inflorescence.

Pentstemoniopsis is undoubtedly a good genus, if we include the vegetative characters, and the structure of the inflorescence in the diagnosis.

EXPLANATION OF FIGURES.

FIG. 1. Chionophila Jamesii. Diagram of the shoot; for explanation see the text.

FIG. 2. The flower, seen from above; enlarged.

FIG. 3. The fruit surrounded by the marcescent calyx and corolla; enlarged.

FIG. 4. The capsule removed from the calyx and corolla; enlarged. FIG. 5. Cross-section of a secondary root; Ep. epidermis; Ex.= exodermis; Ph.=phellogen; X 320.

FIG. 6. Cross-section of inner part of same root; C.-cortex; End.= endodermis; P.-pericycle; L. leptome; V. vessels; X 496.

FIG.

7. Cross-section of inner part of stem; S.-pericycle; P.=pith; H. hadrome; other letters as above; X 320.

FIG. 8. Ventral epidermis of leaf with a stoma; X 496.

FIG. 9.

Epidermis of stem with hairs; X 320.

FIG. 10. Dorsal epidermis of leaf; X 496.

FIG. 11. Pentstemoniopsis Tweedyi (Rose) Rydbg. The flower, sideview; enlarged.

FIG. 12. Lower lip of flower; enlarged.

FIG. 13. The stamens, four fertile, one sterile; enlarged.

FIG. 14. The fruit; enlarged.

FIG. 15. The seed; enlarged.

Clinton, Md., April, 1920.

ART. IV.-Geology of the Muddy Mountains, Nevada, with a Section to the Grand Wash Cliffs in Western Arizona'; by CHESTER R. LONGWELL.

INTRODUCTION.

The area with which this paper is concerned is in southeastern Nevada and northwestern Arizona, as indicated on the accompanying index map (fig. 1). The general region of which this area is a part has more than one strong appeal to the geologist. It is practically unmapped, and hence has the lure of the unknown. The pioneer scientists who traversed the Plateau and Great Basin regions gave only passing notice to the Virgin and Muddy mountains, and the inaccessibility of the region has discouraged later efforts. As a result, a large area still presents opportunities for scientific work of an exploratory nature. Moreover, the location of the region near the edge of the Basin Range country and immediately adjacent to the Plateau Province gives it a critical interest, because the stratigraphic, structural, and physiographic relations of Basin to Plateau must be determined largely by a study of this border zone. Are the rocks west of the Grand Wash Cliffs fundamentally different from those to the east? What is the nature and what the age of structure lines dividing the two provinces? To what extent does the physiography of the region help in unravelling the more recent geological history? These questions concern matters which are not only of interest in themselves, but are fundamental in the larger problems touching the origin and development of the Plateau, the eastern part of the Great Basin, and the Colorado River system.

Previous reports.-The Whipple, Ives, and Wheeler parties did not map the area considered in this report, but made reconnaissance surveys of closely adjacent areas. Dutton included in his atlas of the Grand Canyon district a generalized geologic map of the Grand Wash and the eastern part of the Virgin Mountains. The first representation of the geology of the Muddy Mountains and the lower Virgin Valley appeared in 1903, on Spurr's map of southern Nevada, included in Bulletin 208 of the U. S. 1 Published by permission of the Director of the United States Geological Survey.