Stevenson.] iOO [Nov. 21, 



Jge of these Deposits. 



The basins occupied by the gj'psum deposits are very deep, extending 

 more than 600 feet below the present surface at Saltville ; more than 400 

 feet at ten miles east from Saltville ; and more than 600 feet below the sur- 

 fece in Buchanan's cove — those being the depths to which exploration 

 has been carried at the several localities. These basins must have 

 been eroded at a time when the continental elevation was greater than 

 now or when the drainage was in a very different direction. It may be 

 suggested that they are great "sinkholes," similar in kind to those 

 which occur so commonly in limestone districts. But there is no proba- 

 ble outlet for waters eroding caverns at more that 600 feet below the 

 present drainage lines ; more, the limestones are too far under Pearson, 

 Taylor and Buchanan for their removal to have much effect. The gypsum 

 at those localities rests on Vespertine, between which and the nearest 

 limestones the whole Devonian, Silurian and the upper part of the Cam- 

 bro-Silurian intervene. 



Nothing has been obtained going to show when these basins were 

 eroded. The extent of erosion prior to their formation was very great, 

 for the Coal measures. Lower Carboniferous, Devonian, Silurian and much 

 of the Cambro-Silurian had been removed from the upthrow side of the 

 fault — a very gradual process, as gradual possibly as that by which the 

 fault itself was produced. But nothing can be predicated on this. Geo- 

 logically speaking, the time required for the removal of 10,000 or 12.000 

 feet of rock is comparatively short, as abundantly appears from the enor- 

 mous erosion done in the Colorado area since the later Tertiary and on the 

 Canadian plains of New Mexico since the later Pliocene, where a greater 

 amount of rock has been removed during a period probably no longer 

 than that during which the great faults of Virginia were forming. 



One might at first suppose that the blue clay may eventually afford some 

 clue by yielding fossils. It immediately underlies the Quaternary con- 

 glomerate of the Saltville basin and everywhere rests on the gypsum. 

 But certainly it was not formed at once after the gypsum ceased to be de- 

 posited. The conditions observed on the Buchanan and Taylor properties 

 show that a verj^ considerable thickness of gypsum had been removed by 

 erosion before the blue clay was deposited ; possibly more than 100 feet, 

 the strings or branches of gypsum protruding into the little ravines being 

 remnants which had escaped erosion. In every instance, the blue clay 

 rests on eroded bosses of gypsum and does net invade the deposit to a 

 depth of more than a few feet, the investing material being the red clay, 

 which clearly has a different origin. It is sufficiently evident then that a 

 gap'exists between the close of the gypsum-making and the beginning of 

 the clay deposit that positively prevents any linking of them together. 



But the amount of the erosion and the general relation of the gypsum 

 to the blue clay, with the relation of the latter to the Quaternary conglom- 

 erate, suggest that the gj-psum is not older than the Tertiary ; until some 

 fossils have been discovered, however, the question of age must be re- 



