204 



SIIORTKU CONTRIBUTIONS TO GENERAL GEOLOOV, lf>21. 



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

 inripient v.-ind-blown sand, which is sand blown ont. of 

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

 sands, which are the finer material lilown lieyond the body 

 of a dune. 



Microsmpic cxamiiialiim. — (I) Thin section: Three char- 

 acters are consjjicuous in the thin section of the original 

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

 that is, not produced l)y grindin.? the thin section — and 

 is a character of wind-depo.sited material; "' (h) the angu- 

 larity of most of the grains; (<■) the abseni.e of a reccTgniz- 

 able deposit of secondary (|uartz 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 fpiartz. and tlie rough angidarity 

 of the grains does not suggest secondary growth, whi(h 

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

 solution, ill) Disintegrated sand; The disintegrated 

 material can be studied to greater advantage because it 

 can be immersed in liijuids of different indices of refrac- 

 tion. Thus by immersing in a liquid of index about I."i5 

 the inner structure of the quartz grains and any possible 

 boundary lietween nucleus and secondary quartz ca:i be 

 more readily recognized. Immersing in a liquid of index 

 about 1.05, on the other hand, brings out brilliantly the 

 surface form and texture. Much more could be ol)served. 

 especially in the latter lifpiid , 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. 



(b) 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-looking. as nught 

 be the residt of mechanical wear, c) Under the favorable 

 conditions afforded by immersion in a liquid of index 

 about 1.55, no secondary silicilication could be detected. 

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

 were found, the smallest having a diameter of about 0.025 

 millimeter and being exceptionally perfectly rounded. 

 Such a grain could jinibably be produced only by pro- 

 longed wind a< tion." 



The main problem of fact, in spile of these ol.)servations, 

 still coiK'erns the cause of the i)resent form of the grains. 

 Secondary silicification is often very veiled, and in the 

 al)sence of comparative studies 1 am not prepared to 

 connnit myself as to the po.s.sil)ility of solution having 

 ac-ti'd on the grains. These two factors affect not only 

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

 cohi'ri-ncc of the observations made favors the belief in 

 their mrrecliiess. They lead to the following conclusion: 



The portion of the ('hey<'nne sandstone represented l.iy 

 this aani))le is nothing like an eolian deposit in an arid 

 region, nor even a i>art of a permanent dune area in a 

 humid climate, but merely an accumulation of material 

 blown l)y the wind out of a deposit of some other origin. 

 (.»nly an exteiisixc field study could disclose the main 

 accunuilation from which this, is sei)arate<l and other 

 ]iossible associateil fades and lead to a complete and 

 satisfactory interpretation. From the roughne.ss of the 

 sand grains I would be incdined to assume rather a delta 

 than a beach deposit as the dcjnunant type. The very 



16 Gnldman, M. I., op.cit., pp.2,so-2Sl. 

 1' Idem, pp. L'71-27J. 



small, perfectly roumled grain noted appears entirely out 

 of place as the product of th(> conditions under which the 

 sandstone seems likely to have been formed and must 

 therefore be a.ssumed 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 

 faxored so that only the smallest grains retain their 

 original form; but this seems to me incom].>atil.)le with 

 the uniform size of the larger grains. 



.\s brought out in the paper on the ('atahoula 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 M'hose position in the local 

 section is unknown can be conclusive only for 

 tliat one sample. Tlie foreset bedding of 

 \vliich I liave .seen pictures suggests delta 

 deposition, liut no one has made field studies 

 of the Cheyenne sandstone with the object of 

 determuiing tlie (nigiii of llie sediments or the 

 method of sedimentation, and Mr. Goldman's 

 report is therefore chiefly significant in indi- 

 cating what definite ri'sults might be obtained 

 in this region of an oscillating Cretaceous 

 strand by a careful study of the pro])lem. 



The plants and their method of occurrence 

 conclusively indicate a sparse vegetation, at 

 least seasonal dryness, and accimiulation l)y 

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

 tion, as previously stated. \\Tiethcr these 

 facts do or do not indicate beaches, interstream 

 sand hills, or delta (k'])osits can be determined 

 oidy liy future studies. 



.\GE 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 ecjuivalent 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 sanilstone. » 

 The type localities of the remaining seven 

 species were the Cenomanian of Bohemia (one 

 species), the Ataue beds of Greenland (two 

 species), and the Patapsco formation of Mary- 

 land (four species) . 



Of the so-called Dakota forms in the Chey- 

 enne llora tlie following species were previously 

 known from only the '"Dakota": Cladophlibis 

 dakotensis (.Lesquereux) , Cijcadfosycnmnn line- 



