204 



SHORTER CONTRIBUTIONS TO GENERAL, GEOLOGY, 1921. 



On the other hand, the majority of samples of what he calls 

 incipient wind-blown sand, which is sand blown out of 

 other deposits, have this composition, as do also his lee 

 sands, which are the finer material blown beyond the body 

 of a dune. 



MiirnscoTinc examination. — (I) Thin section: Three char- 

 acters are conspicuous in the thin section of the original 

 rock, namely, («) its porosity, which is evidently priman,' — 

 that is. not produced by grinding the thin section — and 

 is a character of wind-deposited material; '" (h) the angu- 

 larity of most of the grains; {c) the absence of a recogniz- 

 able deposit of secondary quartz on the outside of the 

 grains. In many cases it can be definitely proved, by the 

 presence of inclusions throughout the grain, that there is 

 no outer coat of secondary quartz, and the rough angularit}' 

 of the grains does not suggest secondary growth, which 

 tends to restore crystal form, but. if anything, rather 

 solution. (II) Disintegrated sand: The disintegrated 

 material can be studied to greater ad^■antage because it 

 can be immersed in liquids of different indices of refrac- 

 tion. Thus by immersing in a liquid of index about 1..5.5 

 the inner structure of the quartz grains and any possible 

 boundary between nucleus and secondary quartz can be 

 more readily recognized. Immersing in a liquid of index 

 about 1.6.5, on the other hand, brings out brilliantly the 

 surface form and texture. Much more could be observed, 

 especially in the latter liquid, than the time at my disposal 

 permitted, but I can make the following generalizations: 

 (a) None of the portions, not even the coarsest, reveal any 

 unusual abundance of rounded grains; rather the opposite. 

 (6) The surface of grains is not frosted, as is characteristic 

 of wind-blown sand, nor pitted, as if subjected to much 

 solution, but rather rough and chipped -loo king, as might 

 be the result of mechanical wear, (c) Under the favorable 

 conditions afforded by immersion in a liquid of index 

 about 1..55, no secondary silicification could be detected. 

 id) In the finest size a few very small well-rounded grains 

 were found, the smallest having a diameter of about O.OSf) 

 millimeter and being exceptionally perfectly rounded. 

 Such a grain could probably be produced only by pro- 

 longed wind action." 



The main problem of fact, in spite of these observations, 

 still concerns the cause of the present form of the grains. 

 Secondary silicification is often very veiled, and in the 

 absence of comparative studies I am not prepared to 

 commit myself as to the possibility of solution having 

 acted on the grains. These two factors affect not only 

 the form but also the size of the grains. However, the 

 coherence of the observations made favors the belief in 

 their correctness. They lead to the following conclusion: 



The portion of the Cheyenne sandstone represented by 

 this sample is nothing like an eolian deposit in an arid 

 region, nor even a part of a permanent dune area in a 

 humid climate, but merely an accumulation of material 

 blown by the wind out of a deposit of some other origin. 

 Only an extensive field study could disclose the main 

 accumulation from which this is separated and other 

 possible associated facies and lead to a complete and 

 satisfactory interpretation. From the roughness of the 

 sand grains I would be inclined to assume rather a delta 

 than a beach deposit as the dominant type. The very 



>« Goldman, M. I., op. cit., pp. 280-281. 

 " Idem, pp. 271-272. 



small, perfectly rounded grain noted appears entirely out 

 of place as the product of the conditions under which the 

 sandstone seems likely to have been formed and must 

 therefore be assumed to be the product of an earlier cycle 

 in the history of the grain, unless it is assumed that in a 

 deposition of secondary silica the larger grains have been 

 favored so that only the smallest grains retain their 

 original form; but this seems to me incompatible with 

 the uniform size of the larger grains. 



As Ijrought out in the paper on the Catahoula sandstone 

 there are many more factors that might have been con- 

 sidered, but being limited in time and having no assistance 

 for such work I have selected the few that seemed most 

 rapidly determinable and most important. 



A single sample whose position in the local 

 section is unknown can be conclusive only for 

 that one sample. The foreset bedding of 

 which I have seen pictures suggests delta 

 deposition, but no one has made field studies 

 of the Cheyenne sandstone with the object of 

 determining the origin of the sediments or the 

 method of sedimentation, and Mr. Goldman's 

 report is therefore chiefly significant in indi- 

 cating what definite results might be obtained 

 in this region of an oscillating Cretaceous 

 strand by a careful study of the problem. 



The plants and their method of occurrence 

 conclusively indicate a sparse vegetation, at 

 least seasonal dr\Tiess, and accumulation by 

 the wind. This I am sure was a local condi- 

 tion, as previously stated. Whether these 

 facts do or do not indicate beaches, interstream 

 sand hills, or delta deposits can be determined 

 only by future studies. 



AGE OF THE FLORA. 



Ignoring for the present the general rela- 

 tions and the implications resulting from the 

 age of the overlying Kiowa shale or the sup- 

 posed equivalent beds in Texas, as discussed 

 in the preceding section, and considering only 

 the evidence of the fossil plants, I may note 

 that ten of the 23 species are positively iden- 

 tified as "Dakota" forms and two additional 

 species are tentatively so identified. Four 

 others are peculiar to the Cheyenne sandstone. 

 The type localities of the remaining seven 

 species were the Cenomanian of Bohemia (one 

 species), the Atane beds of Greenland (two 

 species), and the Patapsco formation of Mary- 

 land (four species) . 



Of the so-called Dakota forms in the Chey- 

 enne flora the following species were previously 

 known from only the ''Dakota": CladopMehis 

 dakotensis (Lesquereux) , Cycadeospermum line- 



