PSEUDO FAN STRUCTURE. 311 



line area. A much less proportion can be assigned with any degree of 

 probability to sedimentary formations, while the remainder possess the 

 characters of both classes to such a degree that their origin must still be 

 considered undecided. 



Structure of the Piedmont Plateau in Maryland. 



Description of Sections across the Region. — The outline map (plate 12) 

 placed at the beginning of this article serves to elucidate the physiography 

 of the Piedmont plateau in Maryland, and to approximately locate the 

 boundaries of its semi-crystalline and highly crystalline formations. The 

 structure of this region may best be shown by three generalized sections 

 laid across it from west to east, each about fifty miles in length. The two 

 more northerly of these sections are given here.* The third is made the sub- 

 ject of the next succeeding paper by Mr. Charles R. Keyes. They follow in 

 the main the three lines of railroad which traverse the region (the Western 

 Maryland, the Baltimore and Ohio main stem, and Baltimore and Ohio 

 Metropolitan branch), although the northernmost section leaves the West- 

 ern Maryland railroad where it turns southward, at Finksburg, and is 

 continued eastward to Glencoe, a point on the Northern Central railroad 

 directly north of Baltimore. 



The most striking feature of these sections is their radiating or fan-like 

 structure, and the fact (shown also on the map) that the vertical strata form- 

 ing the axis of this fan follow a direction neither parallel to nor coincident 

 with the boundary between the crystalline and semi-crystalline rocks. These 

 two lines start from the same point on the Potomac (Great Falls), but 

 diverge more and more toward the north. The fan, therefore, while its axis 

 is throughout composed of semi-crystalline rocks, has its western flank made 

 up of the least crystalline, and its eastern flank of the most crystalline 

 portion of the Piedmont region. f 



If the sections be followed from west to east, it will be observed that the 

 oldest formation of known age — the Frederick limestone — emerges from 

 beneath the transgression of Triassic (Newark) sandstone as a series of con- 

 siderably folded beds, which are succeeded on the east and apparently over- 

 lain by carbonaceous and hardly altered shales. These are like those which 

 occupy a similar position above the same limestone farther westward, and 

 may represent the Hudson River horizon. Still beyond there follow wdth 



*The capital letters used in these sections are the same as those already explained for the map 

 (p. 302). For distinguishing the various rock-types, abbreviations in small letters are employed as 

 follows: grn., gneiss; .gr., granite; gh., gabbro; m, marble; sp., serpentine; si., slate or schist; I., 

 limestone; s., sandstone; /. I., Frederick limestone; d., diabase (trap); t., Triassic (Newark) sand- 

 stone; and c. s.. Catoetin sandstone. 



fFor this reason the structure of the Piedmont area in Maryland has no true analogy with the 

 so called fan-structure (Faeherstructur) of the Alps and other great mountain chains, where tho 

 axis of the fan is always the most crystalline portion. 



