THE LOGAN 89 



tion most of the area was covered with a shallow depth of fresh water ; 

 but marine conditions returned during the latest Devonian. 



THE LOGAN 



At the beginning of the Logan the Appalachian land north from Ten- 

 nessee is rising and the whole basin, except along the northern border, is 

 subsiding gently, so that the sea encroaches on the land westward and 

 southward . 



East from the AUeghenies of Pennsylvania the Logan conditions are 

 shown imperfectly, for even the anthracite outliers are at a long distance 

 from the old shoreline. The rocks for the most part are coarse sand- 

 stones, with thin shales and very thin coal beds. This is the condition 

 in northern Virginia, but southward in that state the outliers are found 

 still farther eastward, some of them even in the Great valley and within 

 a few miles of pre-Cambrian. There the rocks are finer and the coal 

 beds thicker, so as to be of local importance, though the coal is usually 

 very impure. Still farther south the coal beds disappear, calcareous 

 rocks appear, and eventually the silicious matter is in the form of chert 

 beds, interstratified with thin limestones. 



Beyond the Pennsylvania AUeghenies sandstone with no coal is the 

 prevailing rock in the most of Pennsylvania, as well as in West Vir- 

 ginia, to probably 30 miles south from the Baltimore and Ohio railroad, 

 meanwhile becoming replaced by shale below until at last, in central 

 West Virginia, the whole interval is occupied by shales, extending down- 

 ward into the Devonian for hundreds of feet, a condition continuing even 

 into Kentucky. This change in character takes place at practically the 

 same latitude as on the eastern outcrop. In Tennessee and Alabama, 

 within this central part of the basin, the conditions resemble those on the 

 eastern outcrop. In northwestern Pennsylvania the lower portion is 

 shale and contains an impure limestone. No trace of this limestone 

 appears in oil-well records of western Pennsylvania, but the records of 

 wells in portions of Harrison and adjoining counties in north central 

 West Virginia note the local development of limestones directly under 

 the " Big Injun " or Logan sandstone. 



Still farther westward, along the outcrops in Ohio and Kentucky, as 

 well as under cover in those states, the conditions are strikingly in con- 

 trast with those of the Devonian. The mass thickens rapidl}^ southward 

 and southwestward in Ohio. For about one hundred miles from the 

 Pennsylvania border it is fine grained above and coarse below, but along 

 the western outcrop for many miles it is distinctly and almost constantly 

 conglomerate, though evidently thinner than at the north. The pebbles 

 are flat, small, and nearly uniform in size. Irregular limestones occur. 



