454 BULLETIN 109, UI^^ITED STATES NATIONAL MUSEUM. 



manner for the use of miners. The floor of the principal bed worked 

 in each district is taken for the surface of the model. 



The purel}^ scientific value of these models and of the underground 

 contour-line maps which accompany them is considerable; for until 

 they were made very crude and incorrect views of tlie complicated 

 structure of each basin were entertained eA^en by those best ac- 

 quainted with it; and a large step has been thu^i made in the theory 

 of plication. 



To carry the theory one stage further a large model (2 feet by 

 4 feet) has been made of the uncovered surface of the Medina forma- 

 tion, No. 4, over an area of about 40,000 square miles; that is, from 

 the Maryland and West Virginia State line to southern New York 

 and northern New Jersey; in other words, from the Blue. Ridge- 

 South Mountain range, across the plicated middle belt of the State, 

 into the slightly waved countrj^ north and west of the Allegheny 

 Mountain. The scale adopted, vertical and horizontal the same, is 

 H,000 feet to 1 inch. The surface of the Medina Sandstone where 

 erosion has spared it is laid bare; and where erosion has gone 

 deeper into the lower Silurian formations, the Cambrian and Arch- 

 ean rocks, a restoration of all up to the top of the Medina has been 

 made, based upon the graphic projection of the curves over the 

 grand anticlinals. This model was made in 1884, but has not been 

 published, because its southeastern border was not satisfactory; but 

 the light which it has thrown on Appalachian structure at large is 

 extraordinary; especially as to the kind, direction, and degree of 

 the sidethrust northwestward, and the relationship of the anthracite 

 region to the South Mountain masses. 



Recentlj^ a local and more accurate model has been made of the 

 district of the Seven Mountains near the center of the State, to show 

 the hunching of one of the great synclinals at one stage of its course 

 across the State. The scale of this model (vertical and horizontal) 

 is 3,200 feet to 1 inch. 



Similar models of the bituminous coal basins of the Pittsburgh 

 district and of the oil-sand group of the western counties, will show 

 by the uncovered surfaces of the Pittsburgh bed and the first oil 

 sand, the general slope to the southwest, and their rise and fall over 

 the anticlinals. 



The topographical maps of the survey are largo and elaborate-, 

 and embody the results of years of instrumental woi-k. The field 

 work v»'as plotted on a scale of 400 feet to 1 inch, and reduced for 

 publication to 1,600 feet and 3.200 feet. The first accomplished wa^ 

 a map of the limonitc-bearing lower Silurian region of Blair a]) i 

 Huntingdon Counties, extended to the coal measures at the cresi 

 of the Alleglieny Mountain. The second finished and jmblished 



