GEOLOGICAL SUEVEY OF THE TEREITOEIES. 337 



SO much charged witli it that it percolates through rocks of sandstone 

 nnder them. As atmospheric action vaporizes and dissolves the bitumen, 

 the exposed faces of the strata are generally whitish, and do not show on 

 the outside the appearance of their composition. But when cut into a few 

 feet deep, the shale are found as hard and as black as caunel- coal, breaking 

 in even fracture without marks of lamination. This has caused a great 

 deal of useless researches, borings, and tunnelings, from unreliable re- 

 ports on the presence of true coal at various localities around Green Eiver 

 Station, From my own exploration of these formations, I am satisfied 

 that they do not have any bed of true lignite. The shales are, however, 

 valuable, and may yield by distillation an amount of bitumen large 

 enough to be remunerative, when this matter becomes available to some 

 purpose in the distant localities where it is found. This bitumen ap- 

 pears to be essentially the result of the decomposition of animal matter. 

 I have looked in vain in the shales for remains of vegetables. In the 

 lowest stratum only, ]S"o. 16 of the section, I have found an obscure im- 

 pression resembling a leaf of grass or a narrow flattened stem, rather 

 referable to some fresh-water plant than to a marine vegetable. From, 

 the thinness of the strata of the Green Eiver group, their extreme 

 diversity, their multiplication, and their compounds, they seem to be 

 the result of deposits in shallow lakes where materials were originated 

 and mixed. These lakes were inhabited by a prodigious quantity of 

 fishes, which, destroyed at repeated periods by drought, have partly fur- 

 nished the bitumen to the shales where their skeletons are preserved. 

 Whenever I had time to search for them, I have scarcely failed to find traces 

 of fish-remains in the numerous beds of bituminous shale which I have 

 examined. It is probably to the periodical drain or desiccation of these 

 lakes, to repeated variations of level in these fresh- water basins, that is 

 due the absence of beds of lignite ; these changes having prevented the 

 heaping, preservation, and slow maceration of vegetables, which are ob- 

 tainable only under the permanent influence of water. The alternations 

 of submersion and drougth, on the contrary, cause a total decomposition 

 of vegetable remains resulting in the formation of mud and clay. Even ani- 

 mal remains, especially small mollusks, are affected and soon destroyed 

 under the same influence. The records attest that in Denmark, some 

 shallow lakes have been thus filled in four years with two feet of cal- 

 careous clay by the decomposition of Characeae and thin-shelled Cyclas 

 and Fhysas. Fossil dicotyledonous leaves have been found in yellow clay 

 shales, near Green Eiver Station, and have been already described. As 

 yet, remains of this kind appear very rare in this formation. 



§ 9. EVANSTON. 



A good description, with fine sections of the lignite-beds of this lo- 

 cality, has been given by Mr. A. C. Peale, in Dr. F. V. Hayden's last 

 report, (1871,) p. 194. I have nothing to add to the observations most 

 carefully made and to the details given on this remarkable deposit of 

 combustible mineral, but wish only to make a few remarks on the dis- 

 tribution of the strata overlying the lignite-beds, especially in regard 

 to the conglomerate formation which top's the hills. The sections of 

 Mr. Peale go to the top of the upper coal-bed. From this, in ascending 

 order, the following strata are exposed in the hill above the works of the 

 Wyoming Coal Company : - 

 22 G s 



