NOD I, SMITHSONIAN EXPLORATIONS, IQ10 21 
this particular shale. Limestones known to be much older outcrop 
so short a distance to the east of this that a great fault or displace- 
ment between the two kinds of rocks is clearly indicated. 
With these facts in hand, the fault was traced for a distance of 
thirty miles north and south, thus again showing that the graptolites 
proved the key to the geologic structure of the region. 
Fic. 25.—Fold in Ordovician shale west of Williamsport, Md. Graptolite 
fauna found at X, where cleavage and bedding planes coincide. Photograph 
by Bassler. 
EXPLORATIONS IN THE OHIO VALLEY FOR FOSSIL ALGAE 
AND CORAL REEFS 
Through the extensive studies of the Secretary for several years 
past, the collections of the National Museum are rich in limestone- 
forming pre-Cambrian algze—a low order of water plants that secrete 
lime or silica. An instructive series of these fossils has been placed on 
exhibition, but in order to show the geologic occurrence and evolu- 
tion of this group of plants it was necessary to supplement the pre- 
Cambrian forms with specimens of more recent age. Accordingly 
