NO. 30 SMITHSONIAN EXPLORATIONS, I9QIl2 51 
Geological Survey and the U. S. National Museum, was to obtain 
material for volumes on the Cambrian and Ordovician strata of 
Maryland, to form a part of the series of memoirs published by that 
State. 
This work was done in such detail that it was possible to map the 
Hagerstown and Williamsport quadrangles of western Maryland, 
embracing all of the Appalachian Valley in that State. The various 
formations making up the great Shenandoah limestone series were 
Fic. 56.—Fossil alga, Cryptozoon, exposed along the railroad near Antietam 
Station, Md. Photograph by Bassler. 
first studied, with the result that eight distinct formations, aggre- 
gating 10,000 feet in thickness, were recognized and mapped. Dur- 
ing the process of the mapping, large collections of both rocks and 
fossils, illustrating all the formations of the Shenandoah limestone, 
were obtained. 
In addition, numerous photographs were taken, showing some of 
the more interesting features of the geology. Two of these are here 
reproduced. Figure 56 illustrates a reef of the hydrocoralline or 
calcareous alga, Cryptosoon, as shown on Antietam Battlefield. This 
reef may be noted wherever the base of the Upper Cambrian (Cono- 
