GEOLOGICAL SURVEY OF THE TEREITOEIES. 445 



perforated by the same; though we had no means of determining how 

 much of the lower part of this sandstone may be hidden in this slope. 



The group of fossils found in the dark indurated clay G is, in several 

 respects, a very interesting one, not only because eveiy species is new 

 to science, and all of them entirely different from any yet found at any 

 other locality, or even in any other beds at this locality, (with ]30ssibly 

 one or two exceptions,) bat on account of their modern affinities. Here 

 we have, from beds certainly overlaid by more than 1,000 feet of strata 

 containing Cretaceous types of fossils, a little group of forms, present- 

 ing such modern affinities that, if placed before any paleontologist un- 

 acquainted with the facts, they would be at once referred to the Tertiary. 

 Such examples as this illustrate the difficulties with which the paleon- 

 tologist sometimes has to contend, and show how very cautious we should 

 be in deciding from the alfiuities of new species of fresh and brackish 

 water types of shells (the vertical range of which is unknown) the 

 geological age of rocks in which they are found ; because species of this 

 kind, from rocks of various ages, often closely resemble each other, 

 while they rarely present such well-marked distinctive features as we 

 see in marine shells from diif'erent horizons. Some of the species ol 

 Physa, Cyrena, I^eritma, &c., for instance, from the clays under consid- 

 eration, closely resemble existing species ; Avhile one or two of Melam/pus 

 present but very slight diiferences from Paris Basin Tertiary species, 

 figured by Deshayes under the name AiiriculcL 



It would ai3pear that the indurated clay containing these mixed types 

 of shells must have been deposited in the form of fine mud, in an estu- 

 ary, or possibly a larger body of salt water, into which the fresh-water 

 shells were swept by streams flowing in from adjacent land.* There 

 were f)robably here, however, during the deposition of all this great 

 group of coal- bearing strata, as during the formation of the far more 

 ancient Carboniferous coals, various oscillations of the earth's surface, 

 because we have every reason to believe that every bed or seam of coal, 

 even if oniy a few inches in thickness, was formed in marshes, by the 

 growth and accumulation of vegetable matter, at or a little above the. 

 sea-level, while we find marine types of fossils through most of the in- 

 tervening strata ; showing that after the accumulation of the material 

 of each bed and seam of coal, there was a subsidence, and a return of 

 the sea. 



Above the horizon of Mr. Carleton's m.ine, we only saw marine types 

 of fossils, though there maybe other beds containing fresh-water shells 

 higher in the series. As already explained, we saw in the sandstone 

 supposed to be the lower part of division 18, in Mr. Carleton's coal-mine, 

 casts of one or more species of the marine genus Inoceramus. We also 

 saw higher in that division, on the east side of Weber Eiver, casts of 

 the marine genus Cardiujn ; while in some thinner beds of sandstone 

 and clays forming division 19, just above 18, there occur, on the east 

 side of the river, numerous casts and shells of a peculiar oyster I have 

 called 0. soleniscus. 



This oyster, which, as already stated, occurs at several horizons far 

 down in the series, is very peculiar, and easily recognized by its unusually 

 narrow, elongated, and generally quite straight form. At the locality 

 mentioned above, on the east^ side of the river, many examples of it 



' It is evident that these fresh--v\-ater shells could not, however, have been trans- 

 ported very far, because, although quite thin, they are not water-worn, but %eem to 

 have been deposited in an unbroken condition. Many of them, however, are found in 

 a crushed condition, evidently from pressure during the consolidation of the clay. 



