706 



SCIENCE. 



[N. S. Vol. II. No. 47. 



of development from art early condition of a 

 V-shaped bar inclosing a lagoon, similar to the 

 bars described by Mr. Gilbert on the Bonne- 

 ville shoreline (Men. I., U. S. G. S. 1890, 

 58), to the stage where the lagoon has been 

 filled and the marsh covered with sand dunes. 

 These sand cusps were not produced by ocean 

 eddy currents as in the case of Hatteras, Look- 

 out, etc. (C. Abbe Jr., B. Soc. Nat. Hist., 

 XXVI., 1895, 489-497). Along the outer shore- 

 line the ocean currents with large radii of curva- 

 ture are eifective, but upon the inner shore 

 line the tidal currents are the more important 

 agents. The terms inner and outer are adapted 

 from those used by Prof. Penck in Morphologie 

 der Erdoberflache, 1894, II., 551. 



An ideal scheme of inflowing tide with its ed- 

 dies is given in the figure. The outflowing tide 



would reverse the direction of flow and trans- 

 portation of shore drift. 



Other examples of similar cusps whose for- 

 mation has been referred to tidal action are 

 those on Coatue beach, Nantucket (N. S. 

 Shaler, Bull. U. S. G. S., No. 53, 1889, 1.3), 

 and Eomney marsh in southeast England (W. 

 Topley, Geol. of the Weald, 1875, 211, 303). 

 This material was presented as a portion of a 

 thesis to be published at a later time. 



MEETING OF OCTOBER 29, 1895. 



Some Features of the Arizona Plateau. L. S. 

 Griswold. Illustrated by stereopticon. 



The district here considered includes parts of 

 northeastern Arizona; the middle portion of the 

 valley of the Little Colorado, the region about 

 the San Francisco Mountains, and a portion of 

 the Grand Caiion of the Colorado, being the 

 localities best observed. 



In general the plateau surface is between 

 5,000 and 7,000 feet in elevation above sea level 

 and strikes one as being remarkably smooth for 

 so high elevation ; there are large stretches of 

 nearly level or gently rolling country, diversi- 

 fied, however, by mesas and outliers with es- 

 carpments rising between 50 and 200 feet, 

 shallow but broad old stream channels now 

 little used and leading to caiions with precipi- 

 tous walls. On the plateau top are numerous 

 volcanic elevations, varying in age fi'ora the 

 young cinder cone to the denuded stock. Over 

 the district silicified wood is well known, occur- 

 ring at the base of a gravel and sand horizon, 

 little consolidated, belonging to late Tertiary or 

 Pleistocene times, and lying with slight uncon- 

 formity in part upon probable Triassic strata 

 and in part upon Carboniferous, the older for- 

 mations being little disturbed. 



The trees now petrified, originally grew to 

 large size, eight or nine feet in diameter for the 

 largest, probably conifers, and perhaps not 

 very diiferent from the forest growth of part of 

 the present plateau. This ancient forest was ap- 

 parently thrown down by the wind, for tree butts 

 are common in horizontal position, while only 

 one was found erect. The gravel and sand 

 covering would seem to have come soon, for 

 only a few have fillings of sediment in hollows 

 or give other indication of decay; the logs were 

 buried at least fifty or sixty feet deep. The 

 weight of the overlying sediments crushed the 

 trees so that the horizontal diameters are com- 

 monly greater than the vertical as they are seen 

 in place. Silicification was probably accom- 

 plished by percolating surface waters, as the logs 

 are distant from volcanic vents, as far as known 

 to the writer ; then no hot water deposits were 

 seen accompanying the logs, and the distribu- 

 tion as seen over many miles and reported 

 much more widely would also militate against 

 the theory of change by hot waters. 



The stages noted in the development of the 

 plateau would begin with a baseleveling of the 



