171 



with now and then the impression of a mollusk. In its extension 

 toward the southwest it becomes rapidly more and more calca- 

 reous, until in the valley of the New River in Virginia we find it 

 embracing a limestone formation upward of fifteen hundred feet 

 in thickness, most of which is crowded with carboniferous fossils. 

 In this district, and as traced further toward the southwest, this 

 upper group presents in the ascending order the following succes- 

 sion of mineral masses. 



First — Argillaceous red and green shales becoming more cal- 

 careous, and in the same proportion more fossiliferous, toward its 

 upper limit. 



Second — The great mass of limestone above referred to, con- 

 sisting of an alternation of compact and often oolitic strata with 

 more argillaceous beds weathering into calcareous shale, and con- 

 taining throughout a great abundance and variety of carbonife- 

 rous fossils. 



Third — Red and variegated shales, with thin strata of lime- 

 stone containing similar fossils ; and 



Fourth — Alternations of red sandstone and red shale with 

 brown, buff, and gray sandstones, the latter varieties predomi- 

 nating toward the top of the series. To this succeeds the con- 

 glomerate and other coarse sediment forming the floor of the true 

 coal measures. 



The lower part of this group, in its most southern outcrop in 

 Virginia, includes a great thickness of gypsum, while the gray and 

 brownish sandstones abounding in the upper portion are often 

 impressed with vegetable remains, and in some instances contain 

 plates, and even thin seams of coal. 



The distinctive features of these two great groups of strata, as well 

 in organic contents and lithological character as in their influence 

 upon the topography of the regions in which they occur, early led 

 the State geologists of Pennsylvania and Virginia to regard them 

 as separate divisions of the carboniferous system, and to designate 

 them severally by the numbers X. and XI. in the numerical clas- 

 sification of their surveys ; titles which, without altering the as- 

 signed limits of the two groups, they have replaced in their later 

 nomenclature by the terms Vespertine and Umbral series. More 

 than twenty years ago they made known the existence and geo- 

 logical position of these formations through the descriptions and 



