eckel.] MARYLAND. 185 
below are believed to represent the full thickness as well as a duplication of some of 
the middle beds as indicated. The Potomac section extends from the bottom of the 
formation to and includes a few inches of the coralline ledge. The 36-foot massive 
Stromatopora bed of the Devils Backbone section is believed to come in immediately 
above this, the other beds of the section continuing upward in the order named to 
the top of the formation. 
The Devils Backbone section, measured along the Huntingdon and Broadtop 
Railroad east of Wills Creek, is as follows: 
Devils Backbone section. 
Feet. 
Helderberg-Oriskany contact 
Concealed 42 
Light-gray fossiliferous limestone, with numerous layers; a very light colored 
chert 22 
Light-gray massive fossiliferous limestone; breaks into rectangular blocks 16 
Shaly limestone 1 \ 
Bluish-gray limestone, breaking into shaly fragments; weathering indicates 
much argillaceous material 18 
Massive Stromatopora beds 36 
Shaly limestone, somewhat nodular 10 
Light-gray massive limestone, with upper part containing layers of light 
colored chert 45 
Thin-bedded limestone; the weathered surface covered with small bryozoans: 1.6 
Dark-blue massive limestone, very hard and difficult to break; upper part 
filled with Pentamerus galeatus 36 
Fine, shaly fossiliferous limestone 16 
Massive, dark-blue fossiliferous limestone 40 
Slightly argillaceous, thin bedded, fossiliferous limestone 14 
Gray, arenaceous fossiliferous limestone, with layers of cherty material 16 
Concealed to bottom of formation. 
Total thickness of exposure at this place 328^ 
The measurements made at Potomac station are as follows: 
Section at Potomac station, Md. 
Feet, 
Upper beds concealed; very massive light gray limestone, with a few feet of 
nodular limestone near the top; coralline layer near the top 95 
Mostly concealed, but sufficiently exposed to show that the bedsare generally 
made up of thin grayish limestones; some massive beds are present 240 
Generally thin-bedded, dark-blue limestone, but with some heavy bed-; 
fossiliferous 1 48 
Thinly bedded, dark-blue fossiliferous limestones, with occasional papery 
shales 92 
Helderberg-Salina contact. 
Total thickness of exposed Ilelderberg 575 
COMPOSITION. 
No analyses of Helderberg limestones from Maryland localities have 
been published; but, to judge from its composition in Pennsylvania and 
Virginia, it will be every where found to contain thick beds of nonniag- 
nesian limestone suitable for use as a Portland-cement material. A 
number of analyses of this limestone from Pennsylvania and Virginia 
localities will be found on pages 281 and 324. 
