taL.] MARYLAND. 183 
If, on the other hand, the purer beds of the Trenton limestone arc 
ntended to furnish the principal ingredient, a clay or shale will be 
eqnired for mixture with it. Surface clays of fair quality occur 
hroughout the area underlain by the Trenton limestones, and sonic 
>f them will probably be found available for use. In addition, the 
hales and slates of the Hudson formation, which borders the lime- 
tone belts, will be serviceable. The composition of some of these 
ludson shales in Pennsylvania and Virginia is given in tables on 
>ages 288 and 322. 
HELDERBERG LIMESTONE AND ADJACENT (LAYS. 
DISTRIBUTION. 
The Helderberg or Lewistown limestone outcrops in Maryland in a 
umber of different belts. Most of these are in the west-central por- 
ion of Allegany County, while several occur in western Washington 
-ounty. The Lewistown limestone occurs only in these two counties. 
ts distribution in Allegany County is described as follows, by Mr. C. 
). O'Harra, in the report on the geology of that county published by 
he Maryland Geological Survey: 
The easternmost and largest area, shaped like a much constricted letter W, lies 
) the east, west, and south of Tussey Mountain, and by its prominent double bii'ur- 
ation makes up a large part of Warrior Mountain and Martin Mountain. On the 
tate line east of Tussey Mountain the Helderberg belt is less than one-half mile 
ide, while the width of the corresponding outcrop on the western side is consider- 
bly greater. Southward, owing to the pitching of the Tussey Mountain anticline, 
:iese bands gradually approach each other until, at a point near Rush, the two 
oalesce. Within less than 1 mile southward the area again becomes bifurcated, but 
lis time, owing to the synclinal nature of the fold, the projecting parts are separated 
y the Oriskany formation, which immediately follows the Helderberg. Of the two 
3uthern Helderberg projections, the one farthest east is the more extensive, and 
icludes within it Flakes Knob, the highest point in the county east of the Alle- 
any Front. This part of the area narrows southward, but caps Warrior Mountain 
d within almost a mile of where the mountain ceases to be a distinct topographic 
mature. The projection lying farther west is much narrower than the one to the 
ast, but continues almost as far south and acts as a capping for Collier Mountain. 
The next area of Helderberg lies farther west and flanks the outcrop of Salina 
round Evitts Mountain in much the same way that the first area does the Salina 
round Tussey Mountain. The bifurcation at the north caused by the Evitts Moun- 
lin anticline is quite like that produced by the Tussey Mountain anticline. The 
Donation continues southward in one long, continually narrowing band to 
within 1| miles of the Potomac, where the Helderberg ending in a sharp 
toint passes beneath the Oriskany to appear again at the roadside by the canal 
here the Potomac has cut entirely through the overlying Oriskany and into the 
lelderberg for a distance of fully a hundred feet. The eastern part of this area 
orms much of the crest and western slope of Nicholas Mountain, while the contact 
ine along the western side is clearly marked by a row of hills extending from the 
Itate line southward. This row of hills reaches almost as far south as does the 
lelderberg outcrop, but finally coalesces with Nicholas Mountain. 
East of Wills Mountain a belt of Helderberg averaging less than one-half mile in 
