180 CEMENT MATERIALS AND INDUSTRY. [bull. 241 
The third and easternmost area of these limestones lies in the eaJ 
central portion of Frederick County, along- Monocac}^ River. Fred 
erick, Adamstown, Frederick Junction, Woodsboro, and Walkersville 
are located on this area of limestone. 
It must be borne in mind that these limestone belts, as abovi 
described and as shown on the map, PL XV, opposite page 314, includ 
two very different types of limestone, one of which is excellent as 
Portland-cement material, while the second is absolutely worthless fo 
that purpose, because of the very high percentage of magnesia tha 
it usually contains. This fact is discussed in considerable detail 
the descriptions of the Lehigh district cement rocks of Pennsylvani 
and New Jersey, and the reader will do well to look over the discus 
sion there given (pp. 284 et seq.) 
As a guide to searching for and recognizing the Trenton limestoni 
which is the one suited for cement manufacture, it may be said tha[ 
the Trenton is usually dark gray to almost black in color, often sla 
in appearance; that it frequently contains fossil shells, but rare 
includes the beds or masses of chert which are so common in t 
underlying light-gray or blue magnesian limestone. 
As the Trenton limestone is, moreover, the higher of the t\j 
limestones, it will usually be found along the contact between t| 
limestone belt and the slates or shales which border it, while the maj 
nesian limestones commonly occur near the middle of the belt 
limestone. 
COMPOSITION. 
The Trenton limestone is almost invariably low in magnesia, and: 
therefore suitable for use as a Portland-cement material. In places 
carries a high percentage of cla} r ey matter, as is instanced by its com- 
position in the Lehigh district of Pennsylvania and New Jersey. 
The analyses below are fairly representative of the composition o 
the Trenton limestones of Maryland. It is probable that careful scare | 5 
along the contacts between the limestone belts and the adjoining slate 
or shales would show the presence of clayey limestones similar to ] 
Lehigh cement rock in composition. 
